Black Chocobo + Materia Cave=Bang**?

[ReloadPsi]

I once did this, maybe you could try it out: give Vincent every single summon that he can carry, then give him HP<->MP. He'll most likely end up with 999 HP, especially with a couple of MP Plus (HP<->MP also exchanges the effects of Materia that increase HP and MP), more than enough if he's in the back row and one of your party members has Cover, plus all his weapons are long range anyway, and summons don't lower attack power if you decide to have him attack physically. Also, try to give him equipment that boosts his elemental defenses so that he doesn't have to worry much about magic.

So why Vincent? Well, if you've the patience, you have him summon a lot during tougher fights, then when he's about running out of them (after all, who the hell bothers leveling them up normally?) his Limit should be charged. Use it, watch his HP grow (Death Gigas is recommended as it'll double his HP to 1998) and watch him continue to be useful. With the amount of robotic enemies and bosses in this game, his lightning elemental attacks will be extremely useful against them.