Well, I can say this much. I have beaten both the Ruby and Emerald Weapons, and while they were hard, they weren't the toughest thing I have ever fought in an RPG.

OZMA, on the other hand, is the toughest thing I have ever faced in an RPG. He was the most ungodly, horrifyingly difficult creature ever. I never beat him, and in all honesty, it doesn't really bother me. Something that hard doesn't need to be beaten.