Find a way to drain his magic reserves, such as using skills based on the succubus costume. You can't take this suit with you, but if you have skills based on this suit, they stay with Celica in the Dark World.
This may become available as part of the story, once you've progressed far enough. Some of the components will be given to you by Selena herself when you visit the Dream World, and others can be found in L's shop. This can be determined by the suit's appearance in the tailor's catalog. See the images added by user Chin_Alancia.