Chac can be beaten in a few ways:
1. The always suggested Catnip + Gunner Mastered Dressphere and a Speed Bracer.
2. Have all three as Berserkers... I used that tactic to beat the giant snail. But have a LOT of phoenix downs on hand. Have them mastered the job and all be over level 80.
3. Use the Highland Winds GG on every girl, so you get First Strike, and the Warrior DS so you can use Delay Buster. Use Speed Bracers so all girls have constant Haste (or you can throw a Choc Wing, but if you use these you won’t need any other accessory, really, and you won’t have to waste turns casting and re-casting Haste) and a Sword Tome to speed up the Swordplay clock.
Then keep hitting the Chac with Delay Buster. I tried this almost a dozen times, and unless the Chac gets in an early hit despite your Highland Winds/passive First Strike, killing one of the girls and forcing you to throw a Phoenix Down, it will never even get a turn. You just keep Delay Bustering and it will keep flinching and taking damage while you hold down X (attack). Count the hits (at level 99 you’ll do 2-3k each hit) and you’ll be able to tell when he’s almost dead. Otherwise, you can almost peg down the keys/buttons and wait until he’s dead. 2.5k (average) damage x 3 girls = 7.5k per turn x 60-70 rounds = 437,850 HP. This might take 45 mins, but it’s foolproof and easy.
This only seems to work with an over-souled Chac, so go to Mushroom Rock Road and fight Gucumatzes until one oversouls, and either Escape, or save with sstate if you’re on an emulator and save after every battle, when one oversouls, just revert to the last save and head for the VI. Chac will oversoul.
Some forums and guides claim you can use Berserker’s Eject, but I had all 3 girls with Rabite’s Foots, Ray of Hope and kept doing it until every one was dead (maybe 4-5 rounds, 12-15 times) and it never worked, and I tried that multiple times. Maybe it does work eventually, but not before you’re dead.
Hope either of those help, if not, let me know