Ultima Weapon is a glass cannon, mostly. He won't do much damage to you with his regular moves if you have good defenses, but he's fast and his attack pattern is random. One of his attacks is notably Light Pillar which deals 9999 damage and you can do nothing to reduce it except use Defend to nullify the damage. So technically there's nothing except chance stopping him from putting out constant Light Pillars leaving you with no room to counter attack. (It doesn't happen though)
On the other hand, a single Lion Heart will kill him instantly. So there's that. Really he should be no problem whatsoever.