Draw a few Blind spells from (I think) a Grat in the Garden’s Training Centre. Cast this upon Diablos, and his only way of killing you (the claw-strike attack) is useless. He’ll only be able to cast Demi and Gravija, both of which will bring your HP down but not kill you, and when you party’s HP reaches 1, your Limit Breaks will become a lot more useful (more hits with Renzo/higher chance of a better finisher, better spells for Selphie’s Slots, more time with Zell’s Duel and Irvine’s Shot, etc.). This is a good chance to use Selphie’s The End, if you’ve never seen it before.
But what I mainly use are the limits. Squalls for beating up, Selphies for the healing. So every time Diabolos uses Gravija, all of them can use the limits. I’m searching or Selphies Mega Healing one and everbody will be fine again.
However my first time throught the game was a bit different, I ended up waiting till I had Cerberus, then I used him to cast "double triple" on all my chars. then I used Rinoa to heal, Squall to attack, and Zell to attack with Magic
you. thats how i beat him. when i was low in HP it just cast demi that way i damage him while my party gets healed. Also i only used Squall renzokuken i always keep him
low on Hp sow i can use his limit when i don’t have aura.