Actually, there is. Ozma has no instant-kill attacks (unlike, say, Omega Weapon in VIII with its guaranteed 9999 damage attack). Meteor's damage is random, as far as I know. It could kill you in one hit, it also could barely do any damage at all. That's how the spell works. It's not an exclusively instant-KO move, nor is it unique to Ozma. Doomsday can be protected against, leaving only Neutron Ring as a guaranteed heavy-hit attack. Meteor's the biggest threat, but it's a wild card at the same time since you can never be sure what it'll do. You just need a bit of luck. But it's not the only thing that makes Ozma difficult, he's also extremely fast and has access to some of the strongest moves in the game, which he can use at random. He's unpredictable and that's what makes him hard. The factor of luck with Meteor just adds some extra tension to the fight, is all.