An easy way to kill mutants is by crouching with a very accurate weapon; usually an automatic weapon to keep your distance from it. When you have to reload, run backwards while watching your radar for any walls behind you. Once you have exhausted your primary weapon, switch to your pistol(s) and keep aiming for the head. Continue this until the mutant is dead, or you only have your knife and grenade. If this happens, locate an area to throw the grenade (for example, a corner) and stand on it to prevent the mutant from getting any points.