You need the Gravity Gun (by Space Guy) and Lightning (by Amade) for this trick.
Spawn a bot or Zombie. Pick it up and hover it over your head. Right click to launch the bot high in the air. Then shoot a bolt of lightning after it. The bolt will hit the bot and it will die of fall damage. Pretty neat, eh? If you hit the bot 1 or 2 times before doing this, the bolt of lightning sent after the bot in the air will kill it before it slams back on the ground. Cool, eh?