Demonic Spiders: The enemy general's squad can turn into this if you are fighting them with a unit that they're strong against. Often your best bet is to simply run away and let your allies deal with them, or find a unit that they're weak against and take control of them.
Ear Worm: Most of the music is pretty good at evoking the emotions of the scene. For example, the music at the tavern is relaxing, while in combat the action music can get you pumped up for combat.
Game Breaker: When a friendly group of soldiers (usually 2-3 squads) are moving towards an enemy base, if you take over the units that don't have a shield icon on them while they're near their town of origin, you will spawn a new batch of troops. Can be useful if you want to try and get a unit type you prefer, or one that's strong against units in the defending town, or just simply want a larger force to attack the town with. However, doing this too much will slow the game down considerably, so you should probably only use it 2-3 times at most.
Death units in nightmare. Like the other warriors games, the strongest one is the one with the most effect crowd control, and these guys take the cake. They're supposed to be a glass cannon that you are supposed to take out quickly, but when you control them they are much heartier, and their three spells cover all range problems. Use fire to take out close range crowds, use ice to prevent an advance from reaching you before you're done charging, and use lightning to target distant units. Aside from the lightning having a small range a set distance from you, if you figure it out, you can quickly turn all foes, except enemies both strong and resistant to them, into ashes.
A small one, but rogue ninja are exceptionally good units so long as they don't run out of knives, even against units they're weak against. Their knife throw, compared to others, is a three knife spread and each one does good damage, and much better when leveled up. due to the insane range of the of the knives, as well as the astronomical destructive power at higher unit levels, even units strong against them, simply spamming them can take out entire platoons of enemy soldiers quickly.
Goddamn Bats: Usually played straight when you run into a group, or several groups, of enemy units that your currently controlled squad is weak against, such as running into axe men while controlling sword troops. Or more likely, archers while controlling mounted units.