Quote Originally Posted by Tavrobel View Post
It's a passive "always active" ability in the weapons. They ignore defense and use a completely different equation to deal damage.
They don't use a different equation at all. They treat the enemy as if it had zero defence, just as if they were under Armour Break (however, Armour Break also nullifies the protection that's bypassed by Piercing, whereas celestial weapons do NOT). Apart from this, the equation is the same, and the final damage is adjusted based on the user's HP(or for Lulu and Yuna, MP). At full HP, the damage is 100% of the regular damage. They don't do any extra damage than a regular weapon to an enemy under armour break. The only exception is Auron's, which does about 130% of regular damage at 1HP.

Because the game wants them to? After all, with increasing difficulty, there comes the burden of increasing damage. Are you looking for some kind of proof? I mean, they do that damage because it wants them to. Games don't follow logic. If they did, I'm sure we'd all be discussing perfect numbers or graph theory instead of suspending disbelief.

Razzia and Camisade are inherently much more powerful than regular attacks. However, their regular physicals should not be doing 99999 at 255 defence and protect. (Even Razzia should be doing about 80000 under this condition, I think)
However, that's a lot more than 9999 HP anyway, and Break HP Limit is very overrated.

Auto-Phoenix is very helpful in this battle. Definately pick on only one at a time, Mindy has the least HP but Cindy is the easiest to hit (but with 255 Luck, this doesn't matter, so go for Mindy). Auto-Haste, and either Ribbon or Stoneproof+Sleepproof as well (that, using Stoneproof and Sleepproof rather than Ribbon, was my armour for all optional bosses except Penance). Unless they all get overdrive at the same time, you won't see Delta Attack again, but once one's dead it's CERTAIN you won't see it.

Quick Hit is your friend.
Celestial weapons are too.