If your having trouble beating your Riku Replacia in Reverse Rebirth mode, use the following trick. Stock your three Nines cards but do not use them. Wait for him to use the enemy card that gives him Slight Lock. Use Darkside to copy it. Go Dark and use the three Nines to do Dark Aura, which will take an entire health bar off. Either attack him or immediately reload. Because you have Slight Lock, you will still have all three Nine cards instead of losing the first one as usual. Do Dark Aura again. After he reaches one health bar, he becomes more defensive and uses a lot of 0 cards. You may not be able to do Dark Aura a third time. However, you can wait until he uses all his cards and has to reload. Then, finish it with Dark Aura or just pummel him into submission.