User blog comment:IamJakuhoRaikoben/Chapter 386: Galaxia Blade, Review./@comment-24467372-20140530205114/@comment-7418318-20140531214832

Well... considering that Mard was defeated when the Spirit King used the light of the stars, maybe he's vulnerable to light magic.

Maybe Fairy Law, Fairy Glitter or Urano Metria might work, preferably all three hitting him at the same time. Law from Laxus, Glitter from Makarov and I guess I don't need to say who's going to use UM.

Or better yet, teach Lucy a new technique, maybe even Galaxia Blade itself.