Greater Sphere never does more than 9325 damage to my party, and I've never had Neslug do anything particularly damaging to it either. Now that you mention it, though, I think you're right about Th'uban or however it's spelled; I do remember it having one attack that completely screwed a character over. I don't think it hit the entire party though, but I could be wrong about that because I haven't fought it particularly often.