Atlasflame being frozen is a feat although I would argue one not relevant to the strength of a real dragon. Atlasflame was basically (if not literally) a corpse when he was frozen by silver. It is by a combination of wendy's magic and natsu's flames that he even got to a spirit-like stage. Its unlikely he was anywhere near his true power in either stage.
Anyways, does it make sense for END as a demon to actually match the sheer raw power a dragon seems to have? Demons are powerful although it does not seem like they necessarily exceed what humans are capable of. It would be very optimistic that mard geer even made it to the sort of strength gildarts or makarov had. Wouldn't end need to be hundreds of times stronger than the likes of gildarts or makarov to actually match the sheer raw power of a dragon? And if end is as powerful as a dragon, then how would he be defeated? The situation would be along the lines of when makarov punched atlasflame... Only grey, assuming he kept and mastered demon slayer magic, would be able to do something. And even then we have to consider that laxus was not able to do much against an actual dragon even being a dragon slayer. So grey would probably need to surpass the sheer raw power we have seen from laxus and gildarts...
I don't see it as demons being that powerful rather its END that is that powerful. I would liken it to how acnologia even as a dragon seems to be on a different level than other dragons.
About sheer raw power, I believe there's using physical might and magical prowess. While the two are certainly linked, what we had was makarov trying to match atlas flame using sheer physical might. Aside from atlsas flame being a bad match up (due to his constant flaming body), it would be quite a feat to be able to match a dragon in physical might. Makarov has a number of powerful spells like fairy law and he can use his light magic in conjunction with his titan mode. I believe utilising magic against dragons is a better alternative instead of trying to fight them on a physical basis.
What I think is that END's magic (curses actually) are powerful enough for that. When gildarts used his crash magic during the timeskip, he was able to destroy an island. I'd say an attack that can destroy an island would leave a mark on a dragon.
Yet, considering how easily zeref killed hades and also how hades defeated makarov, its fair to say zeref is leagues above gildarts. For END to be able to defeat zeref, he needs to be immensely powerful.
I agree that mard Geer wasn't gildarts/makarov level and he is supposedly the 2nd strongest demon ever created. This ofcourse points to demons not being that powerful in general.
However, if END is supposed to be around that level, then he has no chance of defeating zeref. He has to much stronger than that to stand a chance.
There are some things that are a bit inconsistent.
For example, Igneel was in a half dead state, seemingly not having up to half of his original power and yet he managed to match acnologia for some time and even take an arm before being killed. This implies Igneel at full power should be equal or superior to acnologia but we know acnologia confronted and bested them in the past since he is responsible for their weakened state.
Acnologia is supposedly immune to DS magic if I'm not mistaken. He took a head on dragons roar from Igneel as well as other attacks, igneel's injuries were seemingly physical-based rather than magic. We know acnologia has to fall in the future, if DS magic doesn't work, then how would he be defeated?
About END and grey needing to be stronger than Makarov and Gildarts, I do believe they will surpass them in the future. Its common for the younger gen to surpass their predecessors and gildarts has stated on one or two occasions that he has confidence in the younger gen to surpass them.
It might not really seem logical especially since we haven't seen how powerful END is, but like I said earlier, acnologia will be defeated and dragons are supposedly extinct now which means the task falls to humans or END.
