I need help fighting Kou!! My party is Atsuma lvl 15, Karin lvl 14/15, Raigar lvl 14/15, and lord onyx lvl 15/16.. Please help me out! I heard that I should use EP drain dance. How do I get that?

One of the best ways to take out Kou is to go after his knight henchmen. When he gets three out on the field, he can unleash a devastating attack. You may want to level up to about 20, in order to make the battle a bit easier. I was about that level when I fought him, and it helped a lot.

Be sure to make use of the EX Dance to get Atsuma's EX points to full. You need strong attacks that can hit as many enemies as possible. Aim for attacking Kou's minions most of the time, since Kou is relatively harmless without them. However, you should aim to hit Kou at least once during each turn to ebb away at his HP. Hopefully, you will have gotten the palace guard golem from outside the destroyed university. His attacks can hit a very wide area so he is ideal for this battle.
It is a very difficult one, but keep trying, and good luck.