In most games, fighting enemies comes down to two phases. A) Fighting through waves of normal mooks and baddies and making your way through levels, and B) fighting boss villains. Of course, a boss is supposed to be a tougher challenge than a normal mook, but they are also a singular enemy (or at least a small amount), and bosses usually test the player's current ability instead of becoming a tyrannical wave of destruction (at least not most of the time).
In some cases, though, the two stages of play have different levels of difficulty. Sometimes the boss fights will be infuriatingly hard, at least compared to normal play, forcing the player to really get into gear (and creating the impression that the game is just little easy segments in between bosses). Other times, bosses seem easy (sometimes ridiculously so), but the main levels make you cry.
If you find a game like this, congratulations, you've encountered Boss Dissonance.
A Super Trope to Easy Levels, Hard Bosses, Hard Levels, Easy Bosses. If a game is one or the other, place it there.
Compare SNK Boss, Boss Game, Mooks But No Bosses.
Castlevania: Lament of Innocence varies depending on the character you use. With Leon Belmont's defensive abilities and special weapons, levels can be quite challenging, especially when you have to engage in any platforming due to Camera Screw, but the bosses are easy once you learn their pattern (yes, even Death). With bonus character Joachim Armster, it's the opposite - his attacks blaze through regular enemies, most of the platforming challenges are removed (since he has no whip swing), but because he can't block or use healing items, the bosses are insanely hard. The worst is the Forgotten One, who is hard enough for Leon to beat, and nigh impossible for Joachim to beat. While the Orb it drops acts as Leon's Infinity+1 Sword, for Joachim the Orb does nothing at all.
The Legend of Zelda: Skyward Swordzig-zags between the two extremes during the first half. On the one hand, you have the easy Skyview Temple followed by That OneWake Up Call Boss Ghirahim, and on the other, the much more difficult Earth Temple and Lanayru Mining Facility having very easy bosses. There's also, of course, the fairly difficult first battle against The Imprisoned outside a dungeon. A Downplayed Trope in later dungeons, however: Notwithstanding Tentalus, the other bosses are generally on par with their levels of residence difficulty-wise. In fact, it's after the first half of the game when overworld bosses battles appear, further blurring the difficulty curve.
The fifth boss, Mutant Jennifer, is a pain in the ass compared to her long but somewhat easy stage... that is, if you avoid taking the path to the necromancer midboss.
The very short sixth stage, being the most annoying stage in the game because of those evil, constantly-spawning life-sucking fetus monsters, has a fairly easy boss right at the end. All you had to do is keep hitting it, killing the bubbles that form around it in the process. Of course, the hard part is dealing with the bubbles that drop from the ceiling.
The bosses in God Hand actually aren't that hard (Exception to Azel), all of them having easily abused patterns. Now the regular levels, on the other hand...
The levels of the the No More Heroes series are all rather easy with very basic enemies, with occasional issues with mass gunfire. The boss battles were the obvious focus, where all the work was put in, although it varies from boss to boss.
Diablo 2 does both kinds. Act 1 and 2 have bosses that can chew you up in the matter of seconds if you blink. Especially Duriel, who's not only super tough and super fast, but you must also fight him in a small chamber that doesn't even leave any room for strategy, so if you're playing a ranged character, you can kiss him goodbye. In Acts 3, 4, and 5, the Elite Mooks and their leaders that you need to fight before facing the boss are MUCH harder than their infernal masters. Mainly because when facing the boss, all you really need to do is dodge. In hell difficulty, act bosses and superuniques without minions are usually much easier than normal uniques and their minions since act bosses don't get extra boss modifiers and immunities.
Atlantica Online's many dungeons have an odd version of both types. In order to complete a dungeon, each level must be cleared of enemies, including mooks and a boss, and usually a couple of minibosses.
It is level-focused in that if you fail to kill all enemies on a floor within an allotted time, you fail the dungeon. Also, nearly all enemies will group together in 3s, and most are much stronger than their level indicates.
The boss difficulty kicks in around the last few battles involving the boss and minibosses. They have the HP equivalent of over a dozen mooks, often cast spells that damage/debilitate all your mercenaries, and to top it off, have a wall of mooks in front of them and as support.
Oh and the Nation dungeons also throw Escort Mission aspects into the mix.
City of Heroes has examples of both kinds. For example, the Imperious Task Force is rather easy the entire way until fighting Romulus at the end, in which many a team has grinded to a halt while they figure out how to use their skills to defeat him. On the other hand, the Elite Bosses in the Lady Grey Task Force (especially the weakened Hamidon encountered in the 2nd to last mission) are considered to be more annoying battles than the rather straight forward Archvillains who are supposed to outrank them.
Spiral Knights has both kinds in the form of the second-tier bosses. The Royal Jelly Palace a clear boss-focused level, being fairly easy followed by a deeply difficult Damage Sponge Boss that can be a huge pain to deal with, since it regenerates health constantly. On the 'difficult levels' side, the Ironclaw Munitions Factory is absolutely full of traps and high-damage rockets, killing you if your mind wanders for a mere moment, but the Roarmulus Twins at the end are a breeze if at least one person in the party knows the fight.
The Legend of Spyro: The Eternal Night has some killer boss battles and regularly throws regular-and-above-regular enemies at you right in the MIDDLE of difficult fights. One example of this being the final stage of the battle against Skabb the pirate, seen here.
In Mega Man 2 the fandom is split between which is the pushover and which is the utter meatgrinder: Quick Man*
veterans, who tend to relish in self-imposed challenges like no-damage runs, are nailed to the wall trying to decipher his AI pattern; newcomers inevitably tackle Quick Man last and just bull rush him whilst loaded up with Flash Stopper and E Tanks
newcomers are helplessly shredded by his barrage of instant death lasers, while veterans pretty much know the stage by heart
In general, using a robot master's weakness on him makes the battle easier, although the amount the difficulty is lowered depends on the boss and the weapon.
Doc Robot from Mega Man 3 not only has souped up and harder versions of certain Robot Master's stages, but you have to fight him 8 times (2 per level) and he takes the powers of the Robot Masters from 2. Spark Man's stage is easy, but fighting a faster, larger version of Quick Man (already That One Boss in the previous game) is not!
Burst Man's stage is a leisurely swim, followed by a boss that will slam you straight into the instant-kill ceiling of his room.
The Mega Man Zero series is focused on difficult bosses, generally. Stages are SLIGHTLY easier than the boss you have to face off against.
Fan games are almost universally boss-focused, perhaps because creators are willing to put work into their robot masters but treat the regular enemies as the sprite rips they are.
Mega Man And Bass deserves special mention: the game is boss-focused if you choose Bass and a level-focused if you choose Mega Man, despite the maps and the bosses being all but exactly alike.
Successfully maneuvering the stages in Mega Man X 6 is a considerably more challenging task than winning the boss fights.
X8 is more or less boss-focused - every boss even gets to turn invincible at some point and fire a tricky Desperation Attack. On the other hand, it gave us not one but two Ride Chaser stages...
Mega Man 5 has both extremes: the stage from hell with an easy boss (such as Gyro Man) or the easiest stage you've ever seen with the boss from hell (such as Gravity Man).
Legend Of Kay has boss battles that are a relief compared to the evil stage difficulty. You will often find yourself overrun with mooks, all with different weaknesses which makes the situation a clusterfuck. The exception is the second form of the final boss who is an example of unfair difficulty, as the only way to dodge his combo is to use the Breakable Jongs on the outside of the arena to keep away. Each jong can only be destroyed by one weapon type and the types aren't in a logical order E.G. Sword, Claws, Hammer. So you can't just press the cycle button once per jong; you often end up whacking futilely against one with the wrong weapon and falling into the lava.
Blur is a weaponised racing game featuring several boss races. Each boss race is competitive until you pick up your first weapon and blow the boss off the road, usually putting him so far behind that there is no way you can possibly lose unless you are extremely bad at the game.
Zombie Panic In Wonderland manages to hit BOTH kinds of dissonance on the same game. The first world is what you'd expect (i.e.: Easy), but the boss is That One Boss, with attack patterns that change as his health goes down and hard-to-dodge moves. The other two worlds are notably harder, but their bosses are really easy, with their predictable, non-changing patterns and dodgeable attacks. Clearly the dev team took more time to make the first boss than the other two combined.
Deus Ex Human Revolution manages to have both types simultaneously. It just depends on how you actually approach the fight. All of the bosses are trivial if you know which weapons they are vulnerable, too. For example, the first boss can easily be eliminated by throwing the barrels of poison gas lying around or spamming him with the Stun Gun. The Typhoon augmentation also pretty much serves as a Game Breaker for three of the fights. However, this is potentially a very stealth heavy game, some of the most effective weapons against them are not the most obvious choices, for example, the Combat Rifle is pretty awful compared to most other weapons for the boss. It's entirely possible to walk into the third boss with little ammo, ineffective weapons, and if you make a certain story decision being unable to use any augments and having an Interface Screw on top of that. So, if you go into the bosses prepared properly, they are trivially easy, but if you are not properly prepared (which might be likely due to Wrong Genre Savvy and assuming very effective anti-boss weapons are useless since they would be in most games) then they are far more difficult than the rest of the game.
Almost any Fire Emblem level where the boss doesn't move. Most of the time, the boss is effectively a regular enemy at a high level with better weapons, and it's fairly easy to surround them and wear them down. It's even easier when they have no ranged attacks, allowing you to pick them off at no risk. On the other hand, the regular enemies in the game tend to be cannon fodder, while the boss is usually stronger than all the regular enemies in a level combined. Bosses that can actually move are extremely dangerous.
Thracia 776 comes to mind, considering how anticlimactic Beldo is once you get to him. The game is easily the hardest in the series in many fans' eyes.
Super Robot Wars does this with bosses sometimes. Tough bosses are not determined necessarily by their health or statistics, but whether or not they can hit back. This leads to some interesting instances, such as in Super Robot Wars Alpha Gaiden. The penultimate boss is harder, despite being weaker than the final boss simply because the final boss had very limited ammunition for his attacks.
OG Gaiden also has this. You face off with the extremely hard Dark Brain, who has hundred thousands of HP, two forms, regenerate one-third of his HP each turn, can OHKO your character, is generally very hard. Then after that, you go on to fight the final boss... Shu Shirakawa in Neo Granzon. However, Shu has less HP than Dark Brain, less threatening attacks, less regeneration, and all in all, is even easier than Dark Brain.
In Ryusei's Scenario in OG1, you have to fight the R-Gun Rivale which regenerates HP and EN like crazy, has a field that can reduce damage and there are minimal HP/EN Regeneration areas for your characters. The fight after it, however, is easier since the area near the boss is a recovery field. Sure, the boss is on it and also has a field that reduces damage, but when you clear the area and gang up on it, it is all a battle of attrition; If you have SP Regeneration, the battle will be much easier.
Rock Band has some songs that don't fit where it sorts them when sorting by difficulty. On the first one, "Don't Fear The Reaper" appears towards the middle of the songlist for Drummers (there's TWO sections that will kill untrained and trained ones alike, even if the drummers in question can clear Impossible charts). DLC, however, is where this problem is especially evident, as "I.V." (part of the 20 Free songs for Rock Band 2 owners) is slotted as a 4/7 (3 dots) despite numerouscharteddouble-kicks on the drum chart.
Even more infamous are some of the guitar parts. "Constant Motion" in particular is a 6/7, which looks fairly accurate until you hit the guitar solo, which is widely considered to be one of the most difficult (if not the most difficult) one in the entire game, including the 1000+ DLC songs.
It seems Harmonix ranks songs based on their overall difficulty, rather than their peak. This leads to songs such as Afterlife being rated a measly 5/7 on guitar, in spite of sections like this.
Pokemon Diamond And Pearl: With the exception of Flint's slightly diverse team the Elite Four are actually very predictable and easy to beat so long as Elemental Rock-Paper-Scissors is taken into account, but the Champion the player faces after defeating them has a team of the strongest Pokemon in the region with diverse movesets, one of which is both so strong it is banned from competitive play and literally the strongest that a Pokemon of its species could possibly be.
Xenogears gives up entirely on random encounters halfway through and only offers boss battles (although the overworld and previous areas are still accessible should you want to level up). This is due to budget cuts that left the game incomplete.
Saga Frontier is a rare case that can play this one both ways, due to the way the "Battle Rank" system works. Which monsters you run into in a fight are scaled up or down based on how many battles you've done, as well as where you train- Depending on the location, the enemies you fight may be much stronger or weaker than what is "level appropriate". Bosses are set at +2 BR and scale up at different points, so it's entirely possible to go rush headlong into a boss fight you're unprepared for, even though you're capable of slaughtering everything else in the region.
Pretty well justified, though - Full Moon bosses usually have decently long cutscenes between the last save point and the actual battle, while Tartarus bosses don't.
The hyped but forgettable XBOX 360 game Two Worlds featured this due to its item combining mechanic. Duplicate weapons could be squashed together into one weapon with better stats, so those mid-level swords you find dozens of are really all pieces of the Infinity+1 Sword. If you've done any side quests at all on your way to the boss (and it's pretty hard to avoid detouring through the gorgeous countryside), you'll probably kill the final boss in two hits. The Dragon right before the boss? Not so much.