The Sewer monsters. They deal around 50 points of damage PER HIT. Most irritating in level 4, where one exit (assuming there's another one) requires you to go through a whole DEN of those bastards.
The shock drones, after you finally ascend from the sewers. Even on Easy difficulty, they will kill you in about less than a second even when you're at 100% health and armor. On higher difficulty levels, it's a guaranteed One-Hit Kill if they so much as come near you. And they always arrive clung up on the ceiling to strike you where you least expect it, so the best way to deal with them is to stay towards the beginning of the level and not move until you see them coming and then immediately open fire. Also, keep your AI teammates behind you as you don't want to risk a very unnecessary friendly fire situation because you really are going to need them for the next several levels.
Good Bad Bugs: Proper knowledge of a door glitch will allow you to ALWAYS get the first hit on an enemy that's on the other side of a door.
