increase bravery to at least 80+, increase weapon proficiency to at least B rate, use and unlock nodes with slash attack up+ or attack up+ on esper, increase resonance with esper to at least 5, change to monk sub-job, use "store" buff skill. Most things can be done with skip ticket. Of course, you can also max all this to be more stronger. Other way, use lightning esper summon (ramuh) on tyrell.
I maxed out all abilities, got up to lv 89 with Monk subjob. Equipment: Sword of Light +2, Brigandine armor, and Adamantine Ornament for the extra ATK.
Chocobo esper at max lv, with all ATK Up +% and Earth Eater +% nodes, with maxed Hourne Castle VC.
831 ATK & 52 Bravery.
Used buffs for 2 turns, then Luminescent Arc, Store, and finally Scintillating Edge. Got up to 3,300 dmg.
Hope that helps!