I beat Atma with a combination of wall rings, dragoon boots, and the Bomb rage. Everyone had a wall rings, so every time Atma used flare (which was about once every other turn) it would reflect off of my characters and deal 500+ damage to him. Shadow would throw when he got the chance, but everyone mainly acted as a healer. Once Atma entered his "second" attack phase I had Terra and Shadow jump and left locke to die. Gau absorbed flare star because of his bomb rage, and terra and shadow were always in the air when that attack came around. He wasn't difficult in the least bit.

And no, I did not use any cheap tricks if that's what your getting at.