A commercial for Capri Sun V has a young boy being attacked by a Cybear, a giant machine/bear hybrid. The kid manages to kill the "marauding Ursa Machina" by leaping onto his back with a trampoline and stabbing him with the Capri Sun straw.
In Macross 7, a friendly fanservice example from The Movie. One of the giant alien Meltrandi women shares a campfire with the hero and the kid sidekick. The kid has a quite innocent play session in her cleavage (she's at least 30 feet tall, and her breasts are easily the size of beanbag chairs, and the kid is about four years old).
Mazinger Z presents an interesting example because it is an instance of a villain making this to one of his own monsters. In episode 14, to destroy a Mechanical Beast that Dr. Hell had deemed useless, several Iron Masks climb up the several-meter-tall giant robot and plant a bomb in its body.
In Digimon Savers, Masaru and Agumon pull one of these in the final battle against the Big Bad. It doesn't quite work out as planned (they get nabbed by the Big Bad's Combat Tentacles once they reach the top), but it's still awesome.
The battle with Oars in One Piece. Particularly shown when Chopper manages to climb onto Oars's shoulder, tricking Oars into punching himself when he dodges and hides inside Oars's fist, and Zoro uses a special technique to slash his way up Oars's right arm. Kinda subverted by Franky, who just builds a staircase.
Played twice in MÄR with Jack as the climber in both cases.
The first, both characters are normal-sized, but Jack traps her in a beanstalk, forcing him to climb up to her. When his hand gets close to her breast (every indication is that he's going to keep climbing), she screams out that he's a pervert, which causes him to climb back down without doing anything.
The second time, Jack has been shrunk by an attack and climbs his opponent to give his beans a warm environment to grow in (they're on an ice field).
Naruto does this to the Four Tails Bijuu in the Fourth Shinobi War, in order to free him from Tobi's control.
Done briefly in the first Mini Squid Girl segment, with Eiko. In this instance, Eiko is normal-sized and Mini Squid Girl is doing the climbing.
Attack on Titan can be described as "Colossus Climb the Anime Series". The eponymous Titans range from being as large as houses to as tall as a skyscraper, and to reach their weakpoints, humans have to use waist-mounted grappling hooks.
In X-Men, the trope enabled the more melee-oriented Wolverine to pull his weight whenever the team had to fight the Sentinels. That is, if someone who had superstrength wasn't around to serve him up a Fastball Special. Oftentimes, Wolverine is almost more than likely capable of climbing his way up to take them out head on. But downplayed in Ultimate X-Men when Wolverine fights the advanced sentinels and comments that they are smaller and more concentrated, making it easier as he can now simply jump up and attack the head without any climbing at all.
In Issue 20 of Archie Comics' Teenage Mutant Ninja Turtles Adventures, a giant Foot Soldier is unleashed and begins destroying the city. After an initial futile assault is launched on its shins, they climb up in search of a vulnerable spot. Turns out that they only wind up being a diversion so that a fireman-turned-dragon (thanks to an old curio shop owner's powder) can defeat it.
In issue 4 of Earth 2, The Flash is the last hero standing against The Atom when he takes out Hawkgirl and Green Lantern. Flash swiftly climbs up the colossal Atom and pummels his face until he falls over. It doesn't keep him down for long, though.
Played for laughs in Shrek 2 when Puss-in-Boots attacks Shrek (including an Alien-style chest burst, no less).
A human face becomes the location of the Final Battle between anthropomorphic white blood cell Osmosis Jones, voiced by Chris Rock, and evil virus, Thrax, voiced by Laurence Fishburne. Along with Thrax, Ozzy is ejected out of his host and lands on the upper face of Shane (the daughter of Osmosis's home, Frank, and whom Thrax intends to infect and kill after disposing of Frank). During the battle, they plunge into the murky watery depths on the tip of her sclera, avoid being crushed by her oncoming blinking eyelid, and commence battling on one of her eyelashes.
The standard attack of the Nac Mac Feegle (six-inch-high Violent Glaswegians) in the Tiffany Aching novels is to clamber up their opponent and headbutt him with an effect like lead shot.
Similarly, Wee Mad Arthur in Feet of Clay can skitter up a man's pant leg and break his kneecap.
A Rustle in the Grass by Robin Hawdon is a novel about ants told in a Heroic Fantasy style. At one stage the anthill is attacked by a bird and the ant hero drives it off by climbing up its body to attack the bird's eye.
The Mechas in Orson Scott Card's Empire, apparently designed by a particularly Genre Savvy engineer, are nearly impossible to climb on. Our heroes have to get creative to take them out, by pushing on the legs to upset the balance, and then hitting it with two cars. They find a much, much easier method once they link up with the National Guard in the form of AT-4.
This is standard operating procedure for Elementals, 8' tall Power Armor-wearing Super Soldier troopers, genetically designed by the Clans to be anti-Mech infantry. As even the smallest 'Mech is still over 6 meters tall, Elemental teams, operating in groups of five, often enact a Colossus Climb to destroy enemy 'Mechs, mostly to Attack Its Weak Point. Because of the construction of some types of 'Mechs, which often lack humanoid features like hands, this trope is considered a perfectly sensible tactic instead of a suicide run.
However, there are some Inner Sphere infantry units, who are not 8' tall nor clad in armor that can resist 'Mech-scale weapons, who will occasionally try to do this with varying degrees of success. It is possible for simple infantrymen to inflict critical leg damage to enemy 'Mechs this way, and the novels describe at least three instances where a 'Mech was captured or destroyed by infantry making climbs.
Warhammer 40,000: At least one Titan has been defeated this way, by infantry swarming it to overload its onboard troops and weapons.
The Prince's signature move throughout the Sands of Time series is to use his great agility to run up an enemy's torso and vault over them, thus getting into position to whale on them from behind.
Most of the actual boss battles in Prince of Persia: The Two Thrones incorporate acrobatics on the boss itself. In one case, the Prince climbs a monster, and rides him into a wall at the end of a maze.
And before that in Thrones, in an arena he fights a much bigger giant and defeating him involves weakening him, then performing acrobatics to climb, cripple and kill him.
In Prince of Persia: Warrior Within, the Prince faces Iron Golems that could only be hurt by slashing at their ankles until they fell to their knees, then climbing up on their shoulders and stabbing them in the neck.
In the battle against the giant Butcher in Psychonauts, Raz must use the boss's own arm to reach his face for a melee attack.
In Sonic Rush, the (sort of) final boss is a giant mech, and to attack you have to wait until it punches the stage, then run over its arm and shoulder to spin attack its head.
The second area boss of Sonic 3D Blast required you to jump on top of its hands when it attacked the ground, then allow it to pull you back up so you could reach its head.
The Egg Golem from Sonic Adventure 2 had platforms on its back pop out during one of its attacks, allowing Sonic to attack the control device on top. However, in the Dark story's version of events, Sonic reached the top in one jump.
Later in the same game, Shadow has to grind up a rail "support pipe" on the Biolizard's neck in order to reach its back, where its weak spot was.
Inverted at the end of Sonic Unleashed, where Sonic has to save the colossus he's running all over rather than try to destroy it.
In Sonic Rush Adventure, the boss of Blizzard Peaks is a whale. Smack it open, jump inside to an obstacle course, then smack this weak spot at the end for Massive Damage. Literally. While most bosses in this game take loads of hits, only 3 times of reaching this spot (different course each time too) is needed.
The two memorable boss battles from the first game involve this, especially the minotaur fight. While you don't technically climb the Hydra in the first boss battle, you do have to climb the ships' mast to get eye-level with it and attack.
In II, the player climbs on and inside a literal Colossus: the Colossus of Rhodes, and faces Titans so massive their bodies often ARE the stage.
But in III, the battle with Cronos takes the cake. The guy is easily the biggest boss in the series. For reference: your character is smaller than the guy's fingernail, which is more or less 30 foot tall.
El Gigante must be killed by shooting at him for a while, then climbing onto his back and slashing at the parasite controlling the beast as it emerges. Of course, the player can also just shoot the parasite as well, but that doesn't look nearly as cool.
A later puzzle requires you to ride the hands of a giant robotic statue of Salazar to reach the control switches for a bridge. After you raise the bridge, the statue chases after you.
Another computer game that features this is Rayman 3: Hoodlum Havoc, where Rayman needs to run up the villain's arm to reach the magic thing in its back, in the second stage of the final boss battle.
On the first Bowser Jr. boss battle, Mario must defeat a Humongous Mecha called Megaleg by climbing up one of his legs while avoiding homing missiles being fired from all directions.
There's also the Heavy Metal Mecha-Bowser star later in the game; it involves Mario climbing a massive windup toy of death, complete with spikes, flames and energy weapons and dismantling it as he goes.
Possible subversion in the console version of Alien Hominid. When you fight the pudding boss, a downed lamppost leads straight from the ground to the monster's head. If you ever try to climb up the post, it immediately kills you.
Devil May Cry 4 has a big one of these when Dante takes on the Savior in order to rescue Nero. This is combined with Womb Level as Nero takes it on from within.
The boss of the Wind Temple. They are two creatures that can fly, but Link does not fly. Consequently you spend the whole time standing on one or the other.
Multiple enemies require you to become tiny and go inside them to defeat them, including at least one form of the Big Bad.
Two bosses in Twilight Princess don't have to be climbed as much as getting high enough to use the hookshot on their weak point on their back. Cue Link sitting on the boss frantically stabbing said weak point with his sword until he gets shaken off.
Phantom Hourglass also features a variation where you get catapulted on top of the boss.
In Skyward Sword there's the third fight against The Imprisoned, where you need to whack his seal back into his forehead by launching onto him with the Groosenator. A more-classic Colossus Climb is one of two viable strategies for the other two fights, as well.
Originally, this is how the final boss fight against Ganon in Ocarina of Time was intended to be fought, which would have made Ocarina the Trope Maker instead! However, due to camera and frame rate issues, it was scrapped and the smaller-scale version we're all familiar with was used instead.
In La-Mulana, Sakit is immune to ranged weapons (i.e. your subweapons), so you have to wait for it to punch diagonally into the ground, then climb up its extended arm and whack at its face.
An early boss in Star Fox Assault needs to be hovered onto with the landmaster tank.
X-Men 2 in Clone Wars for the Sega Genesis does this quite impressively on its second, third, AND fourth levels. Video games typically struggle with portraying one of the X-Men's major antagonists, the colossal robots known as Sentinels; often they are shrunken down to only slightly larger than human so they make a more manageable obstacle for the player. Clone Wars gets around this by having the X-Men attack a Sentinel maintenance facility while the Sentinels are off-line. You spend two levels fighting your way up scaffolding to reach one of the Sentinel's heads, and one level going inside a Sentinel and fighting a Boss Battle against its reactor core, then running partway back down the scaffolding before the facility explodes from the power overload. Parts of two Sentinels' bodies are visible in the background throughout the levels, and they are true to their gigantic depictions in the comic books and television shows.
The Wolverine game on the X-Box 360 and PS3 has an AWESOME fight with a fully functional Sentinel. It's completely to scale, towering over anything and everything in the game, and you climb it in boh phases of the fight to destroy it. Hell, the second part of the battle has you doing this AS YOU BOTH FALL FROM THE STRATOSPHERE. The game could have ended there, and lost NOTHING.
In Titanfall, opposing Titans can be "Rodeoed": an infantryman can climb on top of the machine, expose the internal machinery, and fire into them, allowing them to directly damage the Titan's hull.
The Super NES adaption of Star Wars Episode V (Super Empire Strikes Back) has Luke — after escaping his snowspeeder before it was destroyed — climb up an AT-AT's leg, work his way up from the inside, and walk across its backside before fighting the head, all in three separate levels! This contrasts from the movie, where Luke instead tethered to its underside, cut an opening with his lightsaber, and threw a bomb inside to destroy it.
The boss Eligor in Castlevania: Order of Ecclesia is a giant centaur-like creature which can only be damaged if the player hits its eye. This can only be accomplished by destroying the jewels on it's four legs, its two crossbows, finding a way to get under it and behind it without being crushed, and finally climbing up to its back and attacking its eye all while avoiding its various attempts to knock you off again. A certain glyph can skip a large part of this process however.
The Ice Titan, Idol Titan, and Dracolich Titan in Lords of Shadow are huge constructs built for war (with the exception of the Dracolich) that are fought almost exactly like the Colossi from Shadow of the Colossus, right down to having to be climbed to strike at their glowing weak points.
Used in the bossfight against the Humongous Mecha Quadraxis in Metroid Prime 2. In the phase 2 of the fight, Quadraxis' head separates from its legs and starts flying around the arena. The player must climb the Spider Ball tracks on the legs, boost onto the head, and then lay bombs into the weak spots in order to damage it.
In Halo 3, two of the Scarab battles basically require you to blast it's legs until it kneels, whereupon the player can jump aboard and fight their way to the glowing weakpoint at the top of the giant robot bug (If you're good/lucky you can defeat it without having to climb aboard, by shooting at it's back until the armor around the weakpoint falls off, but it's tricky). The final battle, against two Scarabs, is much easier because you have an aerial vehicle.
People in the Dragon Age series tend to get really, really awesome dragon-killing animations involving climbing on top of it attacking it while it tosses its neck, flinging you up into the air, and come down right on its neck, hang on through some more bucking, and finally rip its head apart. You get awesome kill animations for ogres, too, but they aren't nearly as awesome.
One of the uberunits for Chaos (indeed, the only one before expansions added the Daemon Prince) is the Bloodthirster, a forty-foot tall winged demon with a giant battleaxe. He kills infantry by picking them up and chowing down on them. However, the Space Marines' primary commander unit, the Force Captain, carries anti-daemon weaponry, and will always beat the Bloodthirster one-on-one. Upon said victory, the Force Captain will crack the demon on the knee with his hammer, climb up onto its back, and whale on its head until it falls.
The Force Captain has a brilliant one in the sequel as well, against the Tyranid Carnifex. Launching himself at the thing's jaw, after avoiding acidic vomit, the Commander holds fast while being trashed around and unloading his bolter into the creature's head — when it finally flings him up in the air, he smashes it right in the skull with his melee weapon, dropping it like a stone. Win.
Deadly Creatures has the final boss, Struggs, with the player controlling the scorpion. This is basically a Crowning Moment Of Awesome for the scorpion as well - it crawls up his legs leg, crawls into his pants, and stabs him in the balls. THREE TIMES. And the second two times involve it avoiding a shotgun and being stepped on, too. The first time, Struggs falls on his ass and gets bitten by a Gilla Monster. The results are also a Crowning Moment of Funny too, every time Struggs gets it in the crotch and screams.
One of the final bosses in Shantae, the gigantic mechanical Tinkerbot, is a miniature stage on his own. You have to find a way into him without getting killed, and then climb around inside him to find and disconnect all his power conduits.
Drill Dozer has a boss who the player actually has to climb around inside to find his weak point.
Mega Man X 4 has the General, one ofthe largest Boss in the series (not counting some of Sigma'sOne-Winged Angel forms). He can detach his hands to reveal cannons in his limbs, and the hands can act as platforms so the player can reach his weakpoint, the head. (However, since X can Wall Jump indefinitely and has the Falcon Armor that does persistent piercing damage at full charge, he can just cling to the left wall and bypass the climbing portion entirely.)
Most of the fighting against the Flame Leviathan battle platform in the Ulduar raid of World of Warcraft is done with a variety of vehicular weaponry, but the demolishers (catapaults) are capable of launching other players onto its back to destroy its turrets. This is necessary in order to send it grinding to a temporary halt that both resets its continuous acceleration and lets all the other vehicles pound on it for extra damage.
Monster Hunter Tri introduces Jhen Mhoren, an Elder Dragon that hunters can jump on if he gets close enough to the Dragonship. In fact, it's necessary if you want to mine scales and ores from it as well as to hit the two weakpoints on it.
Snake has to climb all over one of the giant robots in Metal Gear Solid: Peace Walker. It's functionally equivalent to a giant tank mixed with a battleship: the sucker is covered in guns and if you don't disable at least a few of them, it'll blow you away even as you climb it. In fact, it seems to have been designed specifically to fight off people who would try to climb it.
In Nintendo Land'sMetroid Blast attraction, this is how a player on foot has to fight Kraid. If you're in the gunship, well... It's a freaking gunship, use your imagination. If you're playing with a friend, however, an on-foot player can grapple the base of the gunship to get up faster.
Kraid is generally battled this way in the main Metroid series as well; in the second phase of his boss battle in Super Metroid, for instance, Samus must jump on the spikes he fires from his stomach to reach a platform so she can shoot him.
All bosses in Dragon's Dogma involve you doing this, though it's not strictly necessary to defeat them; once you're strong enough you can kill them with regular attacks/magic.
Battling enemies in the Pikmin series often ends up this way - naturally, since even the smallest monsters tend to be about twice your size.
In the Attack On Titan Tribute Game, defeating the Colossal Titan requires scaling it to get at the nape of its neck. Unfortunately, the hitbox is pretty small relative to the rest of it, and players attempting to slay it are in for a lot of frustration as it moves around. It even has a red smoke shield that kills any player still on its body for a certain amount of time.
In Ben 10, if Ben is forced to go into a fight as the six-inch-tall Grey Matter, this is generally a big part of his strategy.
In Family Guy, while combating a giant monster made of paraplegics, Stewie climbs on the arm to get to the head (Joe).
Happens twice at least in Jackie Chan Adventures. The first time a shrunken Jackie and Hak Foo have to scale the entire body of the giant Tohru. They both end up in his ear and out his nose. The second time shows features a normal sized Jackie and Valmont climbing the hoodie of Jade, who is gigantic in this scene.
Code Lyoko: Toward the end of Season 4, this happens twice with Ulrich and the aptly-named Kolossus. First, to fight William on its shoulder, and then to dispatch the giant itself by striking its two weak spots; Ulrich manages to do this but is crushed by the falling Kolossus.
Once on Chowder, Schnitzel and Chowder had to climb a giant to make a delivery. Not to the giant, to the guy living on top of the giant.
In Transformers Prime, Jack climbs Megatron in order to purge the Nemesis' fuel tanks. Fortunately for Jack, Megatron is in stasis.
Naturally, Bugs Bunny finds himself in this situation — twice in fact, once with Daffy Duck, running into giant Elmer Fudd's ears and ending up behind huge, window-like eyes... then running back down under his clothing, reducing him to ticklish giggles.
In one episode of The Simpsons, over-eager mountaineers try to climb a park ranger — even though all persons involved are normal-sized.
In the episode "Comic Book Heroes", Cathy attempts this with a Giant RobotClown.
Danny does the same thing with the Living Statue of Addison Single.
BattleTech would occasionally depict Elementals doing this against Core World Mechs, using jet packs to jump on before blasting away at them at point-blank range.
New evidence suggests that the "sickle claw" of dromaeosaurid dinosaurs (Velociraptor and the like) was used as a climbing crampon rather than a slashing blade. Yep, that's right. Raptors climbed on top of their prey.
Bugs do this all the time. Who hasn't got ants climbing unto oneself?
Owners of hamsters will sometimes end up as the colossus.
Ditto for owners of cats.
When a baby kangaroo is born (after only a month in utero), it is a tiny thing that must climb up his 6 foot tall mother's fur to her pouch. There, it finds and latches on to a nipple where it just feeds for about 6 months, before detaching part-time.