From The TimeSplitters Wiki

The Behead The Undead Challenges appear in TimeSplitters 2 and TimeSplitters: Future Perfect. The objective of the challenges is to knock off the limbs and heads of Zombies for points. They can be detached using either Gun or Fists. The heads are worth more points than arms.

TimeSplitters 2

• Fight Off The Living Dead
• Sergio's Last Stand
• Day of the Dammed

TimeSplitters Future Perfect

• Brain Drain
• Rare or Well Done?
• Boxing Clever