I have been playing the crucible with 2 of my friends and currently it feels like its not meant to be played other than solo. The loop simply does not work in coop.
1 - Currently if someone dies, he cannot come back in the crucible. I understand that when playing solo, the whole point is to complete it without dying but in this case, it simply means that my friends will commit suicide in order to be able to keep going and we will start it all over again. I would suggest having a pool of usable lifes for the party during a crucible run or something similar.
2 - The map design feels too cramped, with either walls or holes in the way. Again, when playing solo this is all fine but with friends, it make the whole thing hard to enjoy as people will fall to their death or get hit with friendly fire without seeing what is going on.