Nurses:
Use knife, and do [light light light heavy] combos, they will never get a chance to attack
Ferals:
Use pipe, and do a [light light heavy] combo, this will knock it on the ground, walk up and insta-kill it
Schisms:
Use Axe, lure them near a wall, and then do [light light heavy] combos, they will never get a chance to attack
Smog:
Melee isnt a good choice, so pull out a gun and shoot it twice in the lungs. Wait until it breathes in and you see the bright yellow lungs. Shoot the lungs exactly, so it jumps when you shoot it. Two bullets and it's down.
Lurker:
Axe, heavy or light attack, chances are you will lop off it's head. Otherwise, prepare for a dodge then quickly run up behind it and slice it up.
Siam:
Axe, not much strategy here, make sure you are good with the dodge system. Dodge it's swing, roll and slice - but you'll often miss. That's ok, take your time. When it runs away, follow it so when it stops you can get a couple quick slices in (light ones, don't do heavy). The back side is the weak spot. Take your time with this type and you'll be fine.
Order Members:
Pipe, do [light light heavy] combos. You'll miss the first swing, but usually connect the second, then follow up with the heavy hit. Repeat and they won't have a chance at all to hit you. The ones with the guns... probably want to peg them with a gun real quick. Otherwise, same strategy but get up close to them so they can't shoot you. They die in two hits in the head from bullets.
Needler:
Axe, if you know where they spawn from, you can bum rush them and take em out real quick. If not, prepare for it's first strike, dodge then give it a counter. From here, do a quick [light light heavy] combo, this will knock it down. Run up and just do heavy slashes, when it gets up it will get knocked back down, unable to pull an attack off.