I just did battle with Diablos, keeping the Break Damage/Break Hitpoint in mind,
I had basic stats, with 255 str, yet 130 vit, 70 spr etc.
Level 100, Diablos has 3mil HP, and uses Meteor instead of Demi <-- This is a mega FU especially after he uses Gravija, if you do not have max Spirit. (This is it to help discourage levelling up early in the game) Instead of the random Curaga he casts, I changed it to Ultima, and I set the ratios so that Diablos is much slower in his lower levels, but progresses really quickly mid-high in terms of speed (255 speed after level 60 ish+)
He has 255 evade.. Why? He is the devil. And this makes it so that you *must* use limit breaks on this guy, or if you saved him Triple on your Hit-J/Squall's Gunblade, but... You either have to risk being in low health, or it will take you many years to kill him. I managed to succeed with only two characters, for the sake of the test - Quistis and Squall.
Drawing magic: Full-Cure (you're going to need it. BUT, it's unreliable, because sometimes it wont heal *all* your health and will put you in your place shortly after) Meltdown, for obvious reasons, Demi and Slow, I removed Diablos being vulnerable to blind.. HE IS THE DEVIL. He is now vulnerable to Holy, but absorbs all other elements.
I have hopefully designed it in a way that if you're too overlevelled, if you try and stock up on magic, you will essentially be killed, Diablos is mega-fast, each turn is crucial! Similar to Penance in FFX, one mistake could ruin you, I hope, especially early Disc 1!
Furthermore, in terms of drop, if you do manage to kill level 100 Diablos, or saved him for last, you get 100 Luck-Ups.