In the original TimeSplitters, there's a very rare but somewhat reproducible glitch in which a zombie can live without their head, without cheats. If a zombie is on the ground, and exactly when it's about to get up, shoot it and it'll rise without his head. If this glitch occurs, a single attack is needed before it will die permanently. The zombies will not survive any hit, including with punches other zombies or even minor fall-damage, as the game treats them as having zero health.