You can only take 2 others in and do rumble. I just finished mine with help from 2 others on Saturday. It took about an hour. Have a good void shotgun, and any void abilities your character has. There are a couple maps that are better than the others which offer good hiding spots. You hide and have the other 2 run to you, blast them and only come out when necessary or you need ammo. If you start taking more kills be certain to leave and re-enter a different instance of rumble. Once other players figure out what your doing they will come after you.