Knockout City

This has to be two KO finishes with one bomb ball. So you need to either

1)Hit a player with 1 heart who has a teammate also with 1 heart next to him. The ball explodes on contact so make sure it's a hit.
2)Throw the ball so that it explodes when it reaches two players on 1 heart. If you get the timing right it's better that he catches it as he can't move for a short time.

You'll also need the bomb ball to be available of course. The map doesn't really matter.

Me and a teammate set it up for ourselves by getting all three enemies together and taking off 1 heart from all three opponents. Then it was simply a question of waiting till they were bunched up and getting the timing of the throw down so that it exploded just after reaching them. We got lucky in that our third teammate was afk so he didn't 'steal' a KO finish. It would be best to do this in a team of three. Try and keep a bit of distance from the enemies so they don't hear your beeping ball. Smart teams don't stick together when a bomb ball is in play.