Final Fantasy ~ The 4 Heroes of Light. I cannot beat Chaos, I am level 99

I don't have the game, but usually if there's a problem like this, it might be with your team as opposed to your level. I would suggest trying a different team strategy with differenct classes if you can.

I got this from the Walkthrough of FF-4HL.

Chaos strategy:
Lux, Mysterio and Spell Focus'd Desolator attacks are your friends. Chaos
throws out some light and dark element spells, but tends to lean more towards
dark element spells here. If you can squeeze in Amplify for those Desolator
attacks, do so. Levels coming into this fight were between 49 and 50.

Chaos strategy, Round 2:
Hopefully this didn't come as a surprise, so you could bring your Lux user's
AP totals back to full during the previous fight. Start with Lux and
Mysterio, again, but this time, keep an eye on your HP levels, since this form
of Chaos can deal out more damage than the previous one. Desolator, powered
up with Spell Focus and Amplify, are still your best bets at doleing out
damage; I haven't run across a physical analogue that deals six different
~400 dmg hits every other turn, hence Desolator. Chaos throws out more light
spells this time around, so those Holy Shield holders will get more of a break
this time around.

Chaos strategy, Round 3:
You have to beat the right hand, left hand, then face, in order. You can't
even target a different part; likewise, the other parts can't attack until the
previous body part is dispatched. Both the right and left hand parts are
similar to Chaos' second form, just with less HP, but capable of hitting
harder. The face is what's new. The face has an atack called Big Bang,
which is spammable. With Lux up, expect to take around 100 dmg for all of
your characters. If your Black Mage falls, it's going to be hard to revive
them and get them back behind a Lux, but make it a priority if you find
yourself in this situation. Unlike in the previous rounds, your healer may
have had nothing to do and threw out an attack now and then, but that changes
here; keep them healing the whole party, either Lifegiver/Healthgiver Cure/as
or Healthcare X-Potion/Remedy. After losing 60% of the face's HP, it'll
start casting Cura on itself, healing for around 400HP per use. Two of the
six Desolator strikes should cancel that outright, so don't worry too much
about that, as long as your Black Mage is still alive. Just be a little more
cautious here, and when in doubt from having a "free" action, heal.

I do not have this game yet myself, but I will. Hope this helps.