In the earlier games, combine the Shadow Harness with Ma Chao or Zhou Tai for instant win. Even on the hardest difficulty, as long as you don't get one shotted, all generals and masses of enemies can be easily defeat by spamming the attack button and moving around a little. Considerably nerfed in the latter games but still powerful.
In Dynasty Warriors 3, when you encounter a commander face-to-face and their battle taunt cutscene triggers, their "Commander Stamina" activates and, even on Novice, you can barely chip off a few centimeters of their health with every hit. How do you get around this ridiculous defense boost? Pelt them with arrows without triggering the cutscene. Watch their health plummet faster than your army's morale on Very Hard. It should be noted that this does not work on Lu Bu as he always has Commander Stamina. Have fun with that.
The "Steel/Death" element in DW3 killed any non-officer (from the lowliest archer all the way up to Lu Bu's bodyguards) automatically and could do heavy damage to officer characters (a percentage of their current health, regardless of your strength and their defense). Cao Cao, in particular, became nearly unstoppable with this; you had to set the difficulty to Hard to be challenged at all with him, especially if there were stages that required a bodyguard setup with him. And Cao Cao's weapon with that said element was achieved in the Yellow Turban Rebellion stage, meaning it was difficult, but not impossible, to get it in the first stage of his story mode. You could then leave the game set on Hard and probably not be significantly challenged for the rest of his story. It was eventually toned down (as the Vorpal element in 4, Shadow element in 5 and so forth), but still very useful to this day.
Also in Dynasty Warriors 3 (and even up to 5 before it was changed in 6), to quote a certain YouTuber, Gan Ning was capable of a "2 Musou Offense". His Musou Attack not only traveled a good distance, but inflicted so many hits at once that it could instantly break through guards (you saw that right, not break guards, break through) to continue to deal damage for a solid amount. Even if you needed to learn to know how to control the Musou Attack, basically Gan Ning could just 4-shot any single general with 2 Musou Attacks, and he didn't even need Way of the Musou equipped either. 4-onwards had him literally going faster during the attack while also having a different animation that changed the hitbox, but it still didn't stop him one bit until others overshadowed him around 5.
Dynasty Warriors 5. Ling Tong. Spam C4 with any elemental orb of your choice. You win.
The bow moveset for Sun Shangxiang and Yueying in Dynasty Warriors 6. Once the character is fully leveled up, it is ridiculously easy to mow down enormous crowds and strike repeatedly at enemy officers before they even get within range of you.
Despite Lu Bu's quasi-Nerf in 7, he's the only character with a built-in Synergy "seal" in his skill tree to go with the possible Synergy seal on each weapon, which (by being the only possible character who can "stack Synergy 3x") gives him the highest Attack power in the game. Equip two Halberds with a Synergy seal on each, one preferably the Black Dragon Halberd, then use his Switch Attack (which buffs his attack speed temporarily and can also stun an opponent) on a regular basis and proceed to murder all in your path at super speed.
Lu Bu is also overpowered in the sense that he has no less than 3 unblockable moves, 1 of which is an aerial Musou Attack that deals massive damage and the other has a long invincibility frame. Coupling this with his Halberd's speed buff Switch Attack, and he becomes virtually unstoppable.
Created characters in Dynasty Warriors 7 Empires get whatever weapon you want them to use as a first weapon. Yes, that little girl you just created as a Halberd user can now cause massive shockwaves with its EX attack: an invincible Grapple Move, in the form of a one-handed chokeslam. All the Grapple Moves in this game grant a small bit of invincibility while it's being performed, but that one move...
In Dynasty Warriors 7: Xtreme Legends, acquiring a title for officers that allows them to absorb a portion of the damage they deal to enemies as health when they wield Speed-type weapons. If your officers have high enough attack and strong enough weapons, they can keep their health at maximum forever. You can practically steamroll even the most difficult (5 stars and above) Chaos difficulty stages with relative ease, and health recovery items are completely unnecessary. The only way to be challenged with this setup is to play the even more difficult Nightmare difficulty stages, where enemy officers gain free attack and defense boosts, along with Combat Resistance (normal attacks do not make them flinch). In hindsight, it appears this ability was made for the Nightmare difficulty in mind...
In DW7 Empires, any characters with the Evil Main Fame are this, largely due to the fact that it gives you ample amounts of resource incomes and allows the use of ridiculously powerful Strategems. To name just a few: the Explosion Strategem can continuously blow away all approaching enemies and set bases on fire for a good 15 seconds, while the Poison Mist Strategem outright kills any surrounding enemies in less than 5 seconds under any difficulty. Ultimate Might, the crown jewel of the Evil Strategems, takes the cake by boosting your character attributes to insane levels that a few hits is enough to take down even the most fortified base, and anything the enemy throws at you will barely give you a scrath. Oh, and this effect lasts as long as you still have remaining Troop Strength.
The Brave Main Fame characters also have shades of this, as one of the main benefits grants by this Fame is insanely fast Musou recharge rate and movement speed, both of which increase proportionally to the character's Fame level. Coupling these with the powerful Strategems it gives you, such as "Immovable", which prevents any if all damage to be dealt at you for a good 30 seconds, and a bit of skill, and you have your unstoppable Lightning Bruiser right here.
DW7's Curved Sword (Zhou Tai's EX weapon). No, not because of its weapon strength, but when combined with a character with maximum Dash skill, they're capable of using a maneuver that makes them as fast as (or faster than) Red Hare: By jumping, air-dashing, attacking mid-air, then jump-cancel, air-dash, attack mid-air, jump-cancel, repeat ad-nauseum. Not exactly combat-breaking, but will break some difficulties about reaching out some Lv.5 weapons with strict time limit.
Sun Jian is obscenely broken in Advance, thanks to the nice coverage on his moveset and his second charge having a VERY long combo chains (with each hits having a Super Effective Bonus) that makes killing officers easy. Furthermore, unlike most characters who have similarly overpowered offense (namely Xu Zhu, Guan Yu, and Zhang Fei) and/or a similar moveset in general (Cao Cao and Liu Bei), Sun Jian has a standard movement speed, and an easy to control Musou that deals high damage. While a lot of characters can be a killing machine in Advance when you have their attributes maxed out, Sun Jian has a combination of traits that make him stand at the top as the best character in the game.
Zhang Jiao's Shaman Rod weapon in 8. Its C5 attack ends with a fire wave in front of him that DESTROYS anyone caught in it. Even enemies wielding a weapon with the element the staff is at a disadvantage to will get stunned and burned for their trouble. And since it's a Dash weapon, it's best wielded by the most mobile characters in the game. Doing the combo and rapidly air-dashing out of crowds makes the game RIDICULOUSLY easier.
Although it's still substancially overshadowed by Zhang Jiao's own R1 Musou, which comes in the form of a violent storm of fireballs that hit so ridiculously hard it may as well be the DW universe's equivalent of tactical nukes. It is also one of the few Musous that lets you regain control of your character before it has finished firing, so you're free to follow up with another hailstorm of destruction just in case anything survived the first one. Now say it with me:"Fear the wrrrrrrath of the HEAVENS!!"
In Dynasty Warriors 8, each type of weapon has a unique bonus ability. Boosts in attack and defense, being able to cancel into certain moves that would be otherwise impossible, and so on. And then we have the Halberd weapons. When switching into a Halberd, the player gains a HUGE attack bonus, doubles speed (like your character is on fast-forward) and makes all your attacks hit twice for 5 seconds. It may sound not that impressive, but during these 5 seconds you're so fast that you will be able to do two □, □, □, □ combos, wich will kill anyone nearby (filling your Musou and Rage gauges very fast) and deal incredible amounts of damage on officers. And when you switch off from your Halberd, you make a swipe attack that cuts throught all kinds of defenses and immunities (such as Lu Bu being immune to flinching at Hulao Gate) and stuns enyone nearby. By equiping two Halberds on your characters, you can keep the bonus for as long as you need, and keep enemy officers stunned while you tear their HP away. And to boot: you start the game with three Halberds and can equip them on anyone.
In 8, Cyclone is a general Game Breaker because of it always does percentage damage, even if enemies are guarding. This element makes defeating enemy officers absurdly easy and acquiring 5th Weapons a walk in the park, since it activates 100% of the time, so charge-attacks that activate elements multiple times can just fire and chop enemies down like a hot knife through butter. Finally toned down in the expansion by making it only activating if the enemy is guarding, and reducing element activation on many weapons to a single time, but even then, it's still soundly overpowered. Even though it still triggers if guarding, you can still make it hurt REALLY badly using the Trident Tusk's C6 attack (Zhu Rong's Boomerang weapon) and watch enemy HP get ripped to shreds if they guard the combo.
Some skills can become this with the right combination as well, such as the one that increases Power-up drop rate, meaning that the player can just kill a dozen mooks, pick up some axes and wines then proceed to kick the enemy's ass with doubled damage output.
In 8 Empires, the special action "Threaten". This is because it is a non-combat action that can cause the threatened kingdom to forcibly join up giving you their officers, money, army, and resources. It has a low chance of working with a weak kingdom, but usually there will be 1 enemy kingdom that has around 12-20% of being threatened, and the one who posses the fixed strategy have a higher chance, meaning you have about a 25% even with a starting kingdom if you go early enough, and it keeps getting higher as you get stronger. So, once the first threaten works, it only goes downhill from there. It's actually possible to get an all threaten win and never fight at all, so long as you can pay the necessary character to join you and get them to stay.
Xun Yu in particular stands out as the most egregious example, as his Way of Life (Aide to Kings) not only comes with Threaten, but also the biggest stratagem success chance boost. You can forget fighting as soon as you get him (or any self-created character with the same Way of Life) and one territory, and just Threaten your way to unification.