-14 It appears that bronze colossi are able to beat almost any non-procedurally-generated creature in DF, if not several hundred of them, except Dragons. I'll have to test the other, smaller firebreathing creatures at some point.
Dragons, it turns out, are kind of wimpy. Their firebreath is absurdly powerful, but it doesn't take that much to incapacitate them if you can get close enough. And that's not too hard, since their AI seems to like going in for melee attacks, even when their ranged attack is so much more effective.
Same goes for necromancers. Rather than hanging back and letting their hordes of zombies do all the work (and reanimating all not-too-badly-mangled corpses in sight), they prefer to go into the middle of everything where they can get killed.