Do this in Co-op if at all possible. Enemies have trouble dealing with two players and you'll find certain levels a lot less aggravating.
Most of the enemies can be killed with the skills you picked up during a campaign, but here are a couple of strategies for the more challenging floors.
Floor 2: 4 Grenade launching enemies on the parapet, 1 mid-boss enemy (with a shotgun). Use the skull bombs to kill the four snipers- try to wait until they've crouched down before throwing the bomb. And stay mobile! Once they're down, kill the spawning mid-boss (I'd suggest disarming him with a sprint attack and grabbing his gun).
Floor 3: Battle royale. Keep your distance as much as possible- as the enemies will fight each other! Try to pick up a dropped weapon to kill the survivors from a distance.
Floor 4: This is a REAL HASSLE. You'll face 2 regular enemies, 2 snipers (on the raised level) and a horde of squirrels. Keep mobile to avoid the squirrels and use a sprint attack to disarm the enemy with a shotgun. Take the weapon and kill the snipers (two to three good hits should suffice), and then mop up at your discretion. It's imperative that you kill the grenade launcher sniper asap. Note that picking up the weapon can be tricky due to the lock-on interfering with the button command- try aligning yourself so that there are no enemies on screen when you try to pick it up.
Floor 5: One heavy enemy, two grenade launcher snipers and two regular infantry. Grab the dual fish guns and kill the snipers asap. Keep away from the heavy until the two spawning regular enemies are down (the fish guns are VERY useful here) and then grab the warhammer and go to town on the elephant.