Chac sucks, but it's doable. Each of my characters are level 64-68. Oversouled Chac doesn't cast the deadly Heaven's Cataract which should make the battle a lot easier. Basically my strategy here was to get Rikku (as a Thief with Speed Bracer/Cat Nip) in crit so Cat Nip would activate and each of her attacks would do ~20k damage. Stony Glare can't be avoided or prevented with Stoneproof or anything so you're gonna need a lot of softs/remedies. Yuna should Cheap Shot constantly and use items as needed. Paine is slamming Chac as a Berserker and is totally physical attack proof with Evade & Counter. I got really lucky here after 3:00 because Chac just kept attacking Paine to no avail because of this. Basically just keep everyone alive and a Thief with Cat Nip in crit. Chac has ~460k HP. Seriously. CAT NIP NOTE: This is the HD Remastered version of the game, in which Cat Nip is nerfed and causes Slow and Berserk. If you're playing on the original version of the game, equip Cat Nip to a Gunner and never stop using Happy Trigger. I'm almost certain that works for every boss. For more info, the Final Fantasy wiki is a good stop:
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