On the other hand... if you count all the battles you have to fight to get from the save point to him as a part of the battle...
True , at least you can degenerate them though, unless they kill Quistis, then you are dead.

That's why you shouldn't get anything else than Invinsible Moon. That way you'll only have Invinsible Moon and The Canon. This means there's 50% chanse you'll get Invinsible Moon. Also... you should enter the battle with Zells HP at 1 since he'll get more time for his Duell if his HP is low. Well... I've never actually tried this out but I know Omega Weapon pretty well...
Apparently angelo cannon is much more likely to happen if you don't have wishing star. I think the challenge is probably about the same whether you use just invincible moon or both. (I mean wishing star does do 40,000 consistently, so it makes up for a small probability decrease of getting invincible moon) I've tried this quite a bit, (though what is kind of sad is that I didn't know Omega was always level 100 because I have a FFVIII PC guide and in the PC version he levels with you, I got him to the point that I would've beaten him, and he didn't die ) I think I might get a save with the invincible moon setup as well and try them both out. By the way, I don't think invincible moon happens half of the time. I do put everyone's hp to 1, (I was mostly working with a little lower level Zell (level 62 I think) though, maybe when I level him it will be better) plus put zombie, darkness and silence on everybody. (which unfortunately come off after the first invincible moon)