I beat that punk at level 56 in Expert and level 73 in Normal. I didn't even HAVE Ultima Weapon, nor even half of the items needed to Synthesize it. At level 99, he was just a joke. Of course, this statement comes from someone who NUKED Ars Arcanum at every single possible moment, and had 3 Crystal Crowns.

When level 99, it's actually harder against Ansem because you get so over confident that you can pound him, that you let your guard down. this results in his Heartless shield getting you in midair, and never being able to attack because Attack gets converted to Freeze.