[quote=@Masaki Haruna]
BTW, if Revan is Mard Geer or E.N.D level then he shouldn't be one shotted by that magic. Mard Geer took blows from Devil Slayer Gray and still lived. Like having mild injuries or so. E.N.D for sure won't get one shotted.
[/quote]

Gray only hit Mard Geer with Devil Slaying magic once. The other ice attacks were his standard Ice Make.

[url=http://www.mangapanda.com/fairy-tail/410/14]Source 1[/url]
[url=http://www.mangapanda.com/fairy-tail/411/14]Source 2[/url]
[url=http://www.mangapanda.com/fairy-tail/412/2]Result[/url]

First two sources are precisely one chapter's worth of pages apart.

Devil Slayers completely and utterly annihilate Demons. Revan would get mad wrecked by a direct hit.