This is best done with 4 controllers in split-screen, or boosted online with 3 of your friends.
Start up a 4 player Death match game with everyone riding Reaper. Once you find yourself a Remote Bomb (indicated by a light green pickup), place the bomb on the environment somewhere and let it cook (so don't detonate it). Now have all the other players sit right beside it. Once the Remote Bomb is flashing red, detonate it to kill the three Reapers.
To make this a little easier, have your three friends be on the verge of death with only a touch of health left, ensuring a kill when the bomb detonates.