Something I noticed with Chaos (And some other bosses in FFI) is that they actually changed the way enemy AI works in this version.
Normally Chaos would have a 25% chance to attack, 50% chance to cast a spell, and 25% chance to use an ability. That might still be the case, but what's different is that the spell and ability order used to be fixed. If he casts a spell, he would always cast Blizzaga first, then Slowra, etc etc, eventually ending with Flare and starting over again with Blizzaga. Abilities were the same, starting with Blaze and ending with Earthquake. This spreads out his healing and biggest damage spells over his turns to be more manageable, but now he can just cast Flare or Curaja multiple turns in a row if you're unlucky and you're just kind of screwed. OTOH, you can also run into situations like I did in the first Lich fight where he just literally does not attack or cast a damaging spell before he dies (He's supposed to cast Blizzara as his first spell).
I couldn't remember if this was a change made in DoS or not, so I looked up some videos of the fight and he definitely had his normal AI there. Not really sure why they changed it here, but a lot of other bosses in later games have AI scripts built around doing certain attacks in a certain order, so I hope this isn't a preview of things to come.