Well, he still has all of his knowledge in spells as he ever did, but his current level of magical power is solely determined on the potential of the body he's in. It's just that he hasn't ever found a human body that he could reincarnate into that could equal his original body's potential. His original body was almost unequaled in magical power. IIRC, he beat down Altrouge single handedly, which is quite a feat. I think the body he has in AA is his original body, so it should be pretty awesome (even just being a TATARI, which I'm sure he is).
And yeah, wasn't his Reality Marble called Square or something? I mainly just remember how it supercharged his spells, doubling their power.