END was his strongest emotional peak strength or incarnation of his rage (bad version of Dragon Force because Devil Seed had ascendant). He was thinking that Lucy was dead. He lost control. The same way for Grey who was thinking that Jubia sacrificed her life. They were in th same mood and fought to death to a mismatch. Without Erza's intervention, perhaps, it was mutual destruction.
Also, you have to think that END is no pure Demon. He is an hybrid of Human, Dragon and Devil being, and this was this fusion who made him dying. So Devil's Slayer's Magic is not as effective as against an opponent like Mirajane or Tempester for exemple,who are pure Evil body. They were at their climax and even. Natsu destroyed Ice, Grey froze Fire. Their powers counter each other to no way.
Fire Dragon King's attacks are use like ougi, the same for Grey's Devil's Slayer's attack. Grey can't tap into his Devil Slayer's magic too long, because it fulled his humanity's control. Natsu can't tap into Fire Dragon King Mode because of Magic reserve. Attacks are crazy strong and fulled very quickly.
Natsu has more feat and look more impressive due to area's massive explosion but I think we can't underestimate Grey. He was on par with END who made useless Dimaria's Magic and destroyed her. Both, at this moment, were above Neinhart and Dimaria. Feelings of anger and sadness made them crazy stronger.
It's only my opinion, I accept if you don't agree
