I'm trying to kill him right now. Stupid hard type, there's no way to keep
your characters alive through that battle if you don't spend hours levelling up.
Steal his Dark Matter, heal non-stop with Rosa, and use Edge to throw everything you've got at him. You can also try summoning with Rydia if you ever get the chance.
Personal picks? I can't think of any *impossible* bosses, but Seymour Natus did give me a lot of trouble. Vagrant Story's final boss sucked, too...good thing I saved up on Vera items. I've heard things about Kingdom Hearts' Phantom, too.
I killed him. You know what? The secret to this guy isn't teamwork at all. In fact, you pretty much have to strip away all the deadweight. Since the conservation of energy applies to healing spells, being divided equally between party members, anyone that isn' contributng to the battle just ends up sucking up healing power needlessly, so that hp gets low enough that everyone ends up being killed. Kill everyone else before you use the crystal, except Edge, Rosa and Cecil, when Cecil runs out of throwing weapons, have Cecil do shot for shot while Rosa heals. With just the two of them, they're an unbeatable team, full power every turn. So much for team loyalty though. Kinda an anti-climactic secret to winning the final battle though.