Rocket Wars

This can be done in any game mode, and is easiest done with four controllers.

With any player, you'll need to grab the Buzzsaw powerup. It looks like an overhead view of a saw blade. Once you have it, you need to press and hold to charge it all the way up. You then need to release to shoot it and have it hit all three other ships.

The easiest way to do this is to first use two controllers to fly two ships right next to a third ship so that all three are clustered together and orbiting together on a set path. This can be a pain sometimes because of the physics but you should be able to group three ships together. Then, with your fourth ship, flying around staying out of the way of the cluster and wait for a Buzzsaw to spawn. Grab it, then set yourself up near the cluster of ships. Press and hold to charge it all the way and then you can aim as long as needed. Line up the shot to hit all three ships and you'll get a triple kill and unlock this achievement.