From The TimeSplitters Wiki

The Behead The Undead Challenges appear in TimeSplitters Future Perfect and TimeSplitters 2. They involve knocking the Limbs (heads and arms) off of zombies for points, with a Gun or Fists. The heads are more points than arms when knocked off.

(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