Hammerfight/YMMV

Demonic Spiders: Pretty much any enemy can qualify, but particularly the explosives-using NPC's. They don't do that much damage unless on a direct hit, but they throw you around quite a lot (Possibly right into an enemy's strike) while also stunning and blinding you. Oh, and they can aim their cannons independently from their melee weapons, and it's considered dishonourable hitting them should they drop said melee weapon.
- The glowing explosive bees are worse. They explode unless you hit them precisely in the head (and they generally lead with their tails). If you hit their tails (or they hit you), they explode at point-blank range, chopping off about a third of your health. They appear only once in the story, but it's very early in the game when you're just learning to fight. They return in Grim mode, but thankfully you have allies to deal with them.
Hell Is That Noise: That noise that buzzsaw machines make when they're destroyed.
That One Level:
- The Escort Mission during the Silent Rock "hard" path. Bees everywhere--and that's not counting the fact that the mission has a bug that renders it Unwinnable By Mistake even before the introductory cutscene finishes, requiring endless restarting in hopes that it'll actually play!
- Hammerball in the Hammerfighter's League path. The premise would be interesting were it played with teams of two actual players, or even just human AI riders--but instead, you play alone against one (later two) machines that push the ball harder than you, take no damage and do not get stunned by doing so, and are invincible--yet can and will trap and kill you even though your health regenerates during the match.
- The battle inside the Pendulum right before the end of the game. Some of the toughest enemies appear, and it's very dark and hard to see. It's very easy to get murdered right before the end by a random buzzsaw out of nowhere.
  - Try both the Goddamn Bats (buzzsaws) and Demonic Spiders (mechanized versions of the Sakit with either guns or shocky bits, who are impossible to stun and hardly ever stop charging straight at you.)