If Dr. Strange and Dr. Fate are considered to be on the same level then you can definately expect Mephisto to be on a higher level than Neron.
Easily. Why? You will never see Dr. Strange do anything so absurd to Mephisto. He's dangerous on a cosmic scale. Neron has yet to do anything more impressive to me than empower a lot of supervillains to be even more dangerous. In the end he doesn't have near the same powers.
A list of people Neron has empowered. A small list:
Ocean Master
Black Manta
The Key
I know of more but don't feel like listing them right now. It would require more thinking and I'm too lazy to give a @#$% so that's all I'm doing.
Essentialy Dr. Fate = Dr. Strange, and Dr. Strange > Blackheart. Thats the best I'm willing to come up with right now.
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That's Nabu though,big power and experience difference.
Mephisto doesn't rule hell.His realm isn't exactly hell but close.Fate should own black heart though.Strange has whooped on Mephisto in his own realm and Black heart's mephisto lite.
From what I have heard, the DC Norse gods are probably far inferior to Marvel's. Beating DC Surtur would probably not be that much of a feat compared to beating Marvel Surtur. And I'm aware that DC's skyfathers can create universes, but I once heard that the universe DC Odin created was only the size of a solar system. And I think the claim that Mephisto>>>Neron is pretty accurate. From what I have heard, Neron has never done anything to show that he's cosmic or anything. But Mephisto IIRC has battled Galactus, and can whup the Silver Surfer real good. As for Blackheart, I have no idea how powerful he is. I know I heard that he's had trouble with Daredevil in hand-to-hand combat, so I guess he can't be that tough, which is stupid considering that he's the son of the Devil. But I may be mistaken. Does Blackheart have any feats?