From The TimeSplitters Wiki

The Behead The Undead Challenges appear in TimeSplitters Future Perfect and TimeSplitters 2. They involve knocking the Limbs and heads 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