The rasp/osmose method does take a while, but it's safest and assures you'll win. Yeah, that mp draining will take a very long while (my rasps would do about 400 per cast, osmose would refill all my mp which was in the 5-600 area, and if I recall correctly he's got about 55,000 mp), but you can't really die from it. There are better methods, my favorite was using the esper that made everyone jump and see who killed him (and thus ate the ultima), but they all have possibilities of getting you dead. I threw this method out for people who had trouble with him, that's all.
Also, the thing with espers and stat bonuses...yeah, they're about as needed as a 4 man group of ultima casters. By the end of the game, all you really need is one of the 4 people who can use the Atma Weapon and give them hp boosting gear, an offering, and either a genji glove and ragnarok or excalibur, or an economizer and the quick spell. If you can't kill a boss in 2 rounds you're doing it wrong. Hell, I started going through games specifically not giving people espers because I didn't want spells for my trips to the coliseum. Nothing like using terra and watching her blow 5 minutes of her morph ability standing there casting something silly like merton and wrecking herself. I keep kicking myself for giving my copy of ffvi for the gba to my buddy. I was at the end, going through the secret dungeon...5/8 dragons killed.