Painkiller Hell Damnation

Morphing into your demon form requires you to pick up 66 souls (The green floating aura that appears every time you kill an enemy). Upon picking up 66 of these greens balls of delight, the screen will go white and the enemies will begin flashing orange. Simply aim at the enemy and pull the trigger to blow them away.

There are over 400 enemies on this level, so you shouldn't have too much trouble. Just make sure you stand over the bodies of as many foes as possible to collect their souls before they disappear.