Crucible is designed to be more of a team-event.
If you couldnt do any damage that may be either to the immunitys of the monsters or that your timelimit (5 minutes) expired. After the timelimit you cannot deal any damage anymore. Or maybe you forget the lay the lvl curcible small charm in your inventory. Then the monsters dont get damaged too :P
Teleport to the top left side of the map. theres a bigger platform that needs to be cleaned out in under 5 minutes and afaik lvl 9 is the only level where it allows you to teleport.
gl