Ok, so basically, a Slayer minion kills mobs for their loot, and you don't have to do it I'd recommend saving it for IG's, or a more valuable mob, but you can always reuse them. Just place water until all of the mobs flow to a certain point, and then you can place down the slayer minion. With IGs, it's easier just to use lava. Are you on Aztec or Oasis?