You can boost it in cage match, they can die, just don't miss. Make sure you pick the smallest clipped gun of its class, and fire one shot into their leg at a time (if they want The Invincible challenge, then give them a chance to recover from each red screen and don't kill them).
AR: FAL for the easy single shot or Scar-H for tiny clip
LMG: AUG HBAR
Sniper: Intervention for single shot and tiny clip