Ozma is just random. You can have an extremely easy time defeating him one moment, only for him to wipe you out with Meteor on his first turn the next. It's all a matter of luck, which is what makes him so difficult. The other super bosses all have either a set pattern of moves they infinitely repeat or require a specific strategy to beat. Ozma has none of that; he's entirely unpredictable, which can make him tough as hell or a total cakewalk depending on how the dices roll.