StingingVelvet: I can't beat ANY of the "psychos" in this game.
Ah, the dreaded psychos. On my current playthrough, I'm gunning for out and out level grinding simply ignoring the story completely. This puts you in a good position, defence wise, against the annoying buggers. Combo weapons also play a pivotal role in the battles since they have some of the highest damage per hit available. The nailbat is, imho, better then 70% of the other combos, both in speed and durability and maybe even damage dealt. Like tyler said, you have to grind them down.
This worked for me in three separate psycho battles, the first being the toilet one where I let him do the lunging attack, he misses and then I lay into him with the nailbat. After 1-2 hits, I get back, avoid his stab and let him jump into a cubicle again. It gets a little tedious, since you want to beat the hell out of him all in once, but if you hold that back, the bugger goes down eventually. With the crazed chef, I had 4 I.E.D's at my disposal which as soon as the battle started I quickly threw near where he regains health. This helped massively, since it's a one-two combo, explosion, then nails hitting him . With Slappy, I lured him to the ground floor and let him charge straight at me whilst I was near the water bar cave. He kept hitting the barrier, (since I climbed to higher ground right when he was near me) , resulting in him being dazed for a good while. You can either put a few shotgun shells in him, or like me, take a good chunk of health away with the nailbat.
Like I said though, I'm avoiding the story missions and just doing rescue missions, so I'm level 15/16 after 7 hours of playing it. By then you unlock better running speed, attack powers and more inventory slots. Each one helps with the boss/psycho battles.