If you unlocked Mission 93, do it repeatedly in Mission Mode with Lexaeus - he will kill Anti-Riku without getting hit once and effortlessly. In that case, it is recommended to take the Flower Athirst to avoid being blinded by the Neoshadows. Another way is to equip the "Extreme" ring, that gives you 1HP and doubles your Exp, and then spam powerful Limits. Quicker, but for more experienced players. You choose the one that suits you better.