If you're really having trouble with it you can all wipe and shoot them from up top; that way you won't have to deal with the mobs. Just have everyone die; then send a warlock with self res down to activate the checkpoint, die and respawn up top with the group. Otherwise it's really all about everyone finding spots that are protected from the templar, have lines of sight to the oracles, and allow you to clear out the mobs as they spawn.