* DemonicSpiders: The evil spacemen, no doubt. This is because A) They have something approaching a survival instinct when it comes to dodging your shots, but more importantly B) They're the only enemies in the game that regenerate after being killed
** Not only that, but their projectiles fly ''really'' fast, making it nigh impossible to dodge if one is flying head-on at you. They also do a ton of damage, and even if you can stay out of their line-of-fire, they'll still try to fly in and collide with you.
** The hopping robots that constantly spew bullets at you. Depending on where they appear relative to you, they're either not that much of a problem, or an utter nightmare rivaling the evil spacemen.
** The floor and ceiling-mounted robots that shoot lightning when the player gets near them. The player can only get past in short intervals without destroying them, and if they get knocked into the lightning they can very easily get locked into a severe CycleOfHurting.
* GoddamnedBats: Most of the enemies in the Fortresses, to be frank. Most of them aren't that dangerous individually, but their tendency to hang out in groups, combined with your ''very'' short damage invincibility, makes it easy for you to take gobs of damage with little way to avoid it.
** The indestructable turrets that fire homing shots definitely take the cake. No matter how dangerous everything else is, at least you can actually ''kill'' them.
* GoodBadBugs: Enemies in the Air Fortress, especially turrets, occasionally "spawn" pre-destroyed. Given that, with a few exceptions, destroyed enemies in the game do not regenerate, this is pretty helpful.