Originally posted by Damnion:

It's a walking tree so at a guess i'd say fire arrows, or axes. I actually got all the way through several swamps and looted all the iron I needed without ever seeing one. I did eventually find one though (two actually) when I dragged an entire fulling village into a nearby swamp so I could pick them off while they chased swamp critters around. The goblins downed the abominations pretty fast.