So last night, after many runs, many deaths, and many Nightmares, we finally beat Diabolos Prime for the first time. We had 3 monk/warriors (2 75 monks, 1 71 monk with Asuran Fists), 1 red mage/dark knight (for Chainspell + Stun, Stun, Stun!), 1 black mage/white mage, and 1 white mage/black mage (me). ( Here's a guide on how to beat him... )
The black mage only had time for one spell before Diabolos was dead. I only cast 2 Cure IVs and 1 Cure V before he was dead. By the way, remember you can cancel your Stoneskin if Diabolos does cast Sleepga so the poison will wake you up.
Once you beat this badass, go to back to Kerutoto in Windurst Waters to make him your slave (or you could chose one of the other rewards given for beating him).
( Welcome to the playground. Phantasmagoria! )