If it is not a bug it sure seems like it should be.
I was in the Arena Quest. I had Procipinee, Svedd, my Storm Dragon, and 4 archers (good ones).
The Obsidian Golems are immune to the Storm Dragons fire breath and tail sweep. I think the fire breath is a stretch, but okay. The tail sweep makes no sense though. It is just a big club hitting them.
I also tried to use some of Svedd's special. The Obsidian Golems were immune to crushing blow, hobble, and stun (I upgraded and got stun when I played through the first time--Save is prior to getting stun). Again, I kind of understand hobble and stun, but crushing blow does not really make sense either.
PS Using Growth on a Storm Dragon is awesome.
****edit**** added the game save and a few pics.
Because a Storm Dragon fighting two drakes is just fun.
Uh, that kind of sucks
No problem, just make the Storm Dragon bigger