That's awesome thanks. Also another minor point, if building her for my specific strat, please don't give her any camoflauge (for example, don't use Light Veritas TMR, etc).
Diabolos is a tricky boss, and there's a few ways to do it. I'm going to be doing it with a "budget" team using low end dps, so I have to do all the phases without any skipping. Lexa (with a Barb friend) is a nice, easy to build DPS, so that's why I'm using her. It's so I can post a guide for newer players to try to copy that doesn't involve a full rainbow team loaded down with TMR.
The common way to do Diabolos for vet players is to just load up with some holy chainers and push a threshold every single turn, which prevents him from doing his normal spells (which includes an AoE dispel). If you have enough DPS, units like Sephiroth and Orlandeau are much better for that fight. There's a threshold every 20%, so if you can knock off 20% per turn, the fight is extremely easy.
If you don't push a threshold every round, then after 50% he starts using AoE dispel as his first action, which removes all of your dark resist, provokes, and covers. Then he opens a can of beatdown on everyone..... it's annoying. I'm going to be using a passive provoke tank with evade (Ozetta) to handle it. That's the theory at least.... we'll see tomorrow if my idea works or not