K I am trrying to get him but he keeps kicking my ass im wondering what lvl I should fight him at and any easy methods to beat him?

Use demi(draw it from Diablos), and cure every so often.

But does demi kill? I thought it only took a certain percentage of life?

When Diablos' health gets low enough use a limit(Squall's works fine here)

Yep, Demi can never kill you, and neither can Gravija, and that's where this popular little strategy comes from;

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.

I always get him right in the beginning. It is a bit hard because I only got crappy magic, can't even revive my characters in case they die.

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.

in a recent replay I also went at him straight away, but I pretty much used Irvine's method above, I was lucky cos I managed to get "The End" with Selphie quite quickly.

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

Diablos is not sow tough, it's funny how you can cast demi on him and he heals
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.

Yeah, just make sure your characters have the strongest junctions possible for their magic. Draw Demi and then cast it --- don't use the Cast command when drawing because if you do that, he won't cure you. And as was said, once you've weakened him up enough, that's when you let rip with limits or whatever else you need to kill him off.

With diablos I always use laguna,kiros and ward. As there limits desperado blood pain and heavy anchor do more damage then zells duel selphies magic in general and squalls renzokuken.