I have 3...from the same series:
Dark Machinist Child - Timesplitters 2
Alright seriously, even though you kind of see it at the beginning (it looks like a robot kid, approx. 8 years old) it doesn't change the fact that this kid randomly turns into a giant machine with saws and giant screwdriver things stabbing you. Not exactly HARD, but still... made me go "aw wtf"
Creature (Princess) - Timesplitters Future Perfect
You're in a mansion with a bunch of zombies and ghosts and stuff, pretty easy, just aim above the shoulders. Walk around for a bit, killing these things in any way you please, then you walk into a courtyard and BAM! A giant 3 ton...thing...I can't even describe it, begins to breathe fire, throw fireballs, and whack you with claws of doom.
The Deerhaunter - Timespliiters Future Perfect
Same level at Creature, except this time it's in a dinner hall with raining zombies. Once you dispatch all of them the friggin mounted deerhead JUMPS out of the wall and chases you down like a drunk guy after a pinata.