Diabolos has high resistances towards everything except Slash and Light. This can be an issue if your units don't have any resistance penetration, or any form of lowering them. Diabolos has a lot of strong dark magic attacks, and they deal damage in a wide area, and the fight is only over when you defeat all the enemies (not just Diabolos), so it's highly recommended to bring a magic tank, preferably not a Light unit too. Be sure to not stay close to your magic tank to avoid taking area attack.
Diabolos starts the fight at the highest point in the south area, and if you don't use the "initial placement" option before the start of the fight, most of your units will be at the lowest point of the area in the center, and it's a bad place to try and get out, or even reach the Diabolos. It's highly recommended to change your units' initial placement to the upper areas, to either lure Diabolos to an easier place, or leave your one and only magic tank down there.
After a few turns, 7 Cursed Bones will spawn at different places. Their resistances are also pretty big for the most part, except for Strike, Magic, and Light.
If you have plenty of strong Light units, you might not need a physical tank at all in the fight. Have your light units be close to where the skeletons will spawn, and if your damage is high enough, you can easily one shot them before they get to act. Otherwise, having a physical tank can make it easier to attract all the Cursed Bones.
Magic tanks can get Diabolos' attention, and save you a lot of team heals by just focusing on 1 unit. Unless you have really strong Light units that can burst Diabolos down very quickly, it's recommended to bring a magic tank.
Rain can increase hate through a self buff. This is a huge deal since it means he doesn't need to use AP and find a target to use someting like Taunting Spell.
Even though Elsirelle is a Light element, she can also deal some decent damage and tank some hits because of her spellblade sub job. Though, rotating between attacking and using defensive buffs + Taunting Spell can be an issue. If you bring Rain to the party, then you can just make Elsirelle into the damage dealer.
All of the enemies are Dark, and Sakura has SPR penetration buff, Dark element resistance buff, and a lot of offensive spells to deal with the Cursed Bones and Diabolos.
Yuna deserves a mention too if you have her. She can deal a lot of damage to the enemies, heal, and revive skills too. With the Green Mage sub job, she can use Imperil , Deshell, or Deprotect so your units can deal more damage.