I've been messing around with arena mode for a while, and it seems that bronze colossuses (colussi?) seems ever so slightly unkillable without magma. I set up the following fight in arena mode:
1 bronze colossus
vs
26 hydra, 10 ogres and 3 dwarfs (grand master hammer/armour/shield/dodge/wrestler/fighter, fully decked out in adamantine)
The colossus won, without a scratch.
I promptly set up a second test with the same colossus up against 10 dwarves (grand master axe/armour/shield/dodge/wrestler/fighter, fully decked out in adamantine) at which point the dwarves promptly chopped off the colossus' hands and feet. Fight over you might think, but no, this colossus then went on to break a dwarf's hand almost immediately (don't ask me what it hit him with).
The fight is still on going, its been about 40 minutes so far and the colossus has lost both hands, both feet, both ears and its nose, all of its other body parts are red and its still fighting. Despite its injuries, its killed 4 of the dwarves, and injured all but one of the others to varying degrees.
Has anyone discovered a magmaless way to kill one of these yet? I'm just about to set 2 colossuses off against each other in a separate bit of the arena, to see if either of them can kill each other.
Update - while I was typing that last part, the colossus got another kill.