Diabolos is is weak to slash type and light element, so unit's like Cecil, Robb, and Elsirelle works best here, especially if you have a full team of them so you can chain.
Ramza is the ideal unit for the raid since he can deal a lot of damage through his banish blades that has dark killers with it. The spellblade subjob also gives him the tankiness to survive Diabolos' magic attacks and take the enemy's focus by raising hate.
Consider turning off some of Cecil's tanking abilities since Diabolos attacks mostly with magic. Focus on more offensive setup to make him more of a damage dealer.
Robb is a fairly old unit, but he's quite viable and can still do a lot of damage if you happen to upgrade him to EX.
With only a 5% magic resistance, strong light magic attackers like Yuna and Sakura are still very strong.
Yuna is the ideal unit to bring for this raid since she can deal high damage, or be a support with the green mage subjob to inflict Imperil. Having her on the party can also save a slot for the bonus multiplier, since she's part of the "Large" bonus.
Other strong damage dealers that are part of the raid bonuses are still good options, especially if you have VCs or gears to help with a mono element.
Ice units are the recommended team of choice that's not light since Auron is part of the "Large" bonus. They can deal a lot of damage since Mediena's LB can reduce ice resistance, and Gilgamesh can reduce slash resistance with Kotetsu. Both are powerful debuffs that can really boost your overall damage.
Tidus is a free and strong unit that is definitely still viable for the raid. If you managed to raise him up to 120 and get Jecht Shot, combined with the damage cap up from one of his EX board, he can potentially hit over 10k each attack. However, Tidus may be quite reliant on Illusory Bell so he doesn't run out of AP.
Other water units can help for the water chains. Ildyra has healing for some sustainability, while Creysse has esper killer from her sniper subjob.
Flan Princess starts with no elemental resistances when the fight starts, but it has an ability to raise its resistances to all elements except dark, so units like Garvall is ideal here.
Yuna can still work here since she has Ruin from green mage subjob, which is an elementless magic attack.
Diabolos Stats & Resistances
Max Level Stats
Accuracy Up, Hate Up, HP Debuff Res+999%, SPR Debuff Res+100%, AGI Debuff Res+999%.