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.
A fairly simple one. The game recommends switching regularly between unit types as to avoid running into problems...however if you instead chose Men At Arms as your squad type and focus purely on swords, the level for it quickly shoots up to the point your overleveled for any point in the campaign, your troops, already some of the hardiest all around infantry in the game, are virtually invincible provided careful use of the Tackle ability to countercharge cavalry. Simply buy a large number of speed enhancing items to help get around [you'll have far too much money anyway] and you'll find that you can take the time to take towns you're not required to during missions, and which side controls towns is semi persistent, making future missions in those regions even easier if you choose to focus on one side or the other. Story missions will change towns as needed for their scenario, which means the few you're forced to only join one side or the other will still be doable, with your massively overleveled unit and piles of items and recruits. And this will snowball uncontrollably, as the game, detecting your lower level skillbooks, will still mix in lower tier missions as well for your options, allowing for quick cash grinding.
Gun units. They require a few quests on the French side to gain, but once they start showing up on the battlefield, you will quickly dominate almost any other enemy unit. Their attacks are similar to archer attacks with an aimed shot, a quick fire where they all fire in the general direction your character is facing, and a buff spell to increase their damage. Even their regular default attack consists of the troops firing randomly without that pesky delay of having to reload very slowly. Though they are vulnerable to some troop types, their only real weakness is that they're somewhat slow due to being on foot. On the Nightmare campaign however, there is no day/night cycle, so you can take your time decimating demonic forces. Even dragons, which are usually a tough fight for melee units, can quickly fall to a sustained barrage of bullets.
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.