Look it up on the wikia (Ya i'm stuck too. He's utterly impossible to beat)
Depends which one, first one is easiest, takes a while but target the armour and swich to Sentinel, Medic Synergist save cure for lightning alone when the speacial attack is about to happen, slowly wear him down.
Second time, mystic tower, Delta attack and Combat clinic, all you need. I would try and Max crystarium first. Imediatly swhich to combat clinc after luaghter. If he cast doom, relentless assult him, use renew if you desperately need ot hela, once used up quickly combat clinic, then pound away. I hated this one.
THird one, i would do same as before, but max out using the wladsalius and stuff behind, 32000 points per kill. Good luck, hope i helped.
It's pretty simple. STRIKE THE WEAK POINT FOR MASSIVE DAMAGE!