The arena is totally broken. Scrap it and start again. It is the only fluid part of the game and its a total drag facing the same teams that are indestructible over and over.

I suggest you remove the penalty for defeat.  Try and fight Martyr and if you lose so what, at least you wont need to worry about being demoted. 

I agree unkillable is pretty lame especially when it can be applied almost continuously during the fight. Removing the buff is an option but some have it on such a short CD that it's almost always available or up.