Splody

It’s easiest to do this with 3 controller (or 2 controllers and a Vita), but it can be done with bots. Set up the game in Advanced Options and set Postmortem as "Ghost Bombers". You will need at least 3 players/bots in a match to earn this. When you are killed, you return as a ghost and can kill others. Let one of your controllers kill your first controller, and when you return as a ghost, kill the player that killed you. If you're playing with bots, as a ghost, you'll have to kill the bot who killed you.