iirc you are level 12, so your stoneskin would be 5/+2 which isn't too hard to breach *shrugs* I guess they're just equipped with magical weapons. I daresay they are modified in Layo, not sure on if their level 1-8 immunity would mechanically effect the flame weapon/darkfire, but I think they have DR too, if that's the case the cheetah may not be the best bet, adamantium something would help you a lot there.
How big are the spawns? Mind you wouldn't be a good idea to solo those things until you're approaching 20 at least.