Almost impossible. With ammo belt, it still doesn't have enough ammo to get 20 kills. Only way is ammo salvage which basically forces one to be aiming for headshots and achieve this on only certain maps
Probably the easiest since you can kill enemies that can't even see you. Your chance of getting killed by someone is quite slim when there's a nice fat wall between you and him; Instead of having to get close to someone with the Warp pistol or Deatomizer.