User blog comment:IamJakuhoRaikoben/Chapter 518: Master Enchant, Review./@comment-5303512-20170126232425/@comment-5303512-20170127140627

Remember that Gray was a Devil Slayer so his powers were close enough to help in the battle, also Invel was deliberately getting Gray to become strong enough to face END. Also Slayers can incorporate the strengths of other wizards with the same elements into their own.

But when a battle between two equally strong characters in raw power, it comes down to a matter of who's feelings are stronger.