meh. warlock beat mephisto down in hell and wouldn't even ENTER the battle against odin in asgard. ss HAS overpowered mephisto in hell (i've showed the scan a couple times in the past -- prove to me mephisto 'let' him win . . .) though mephisto has also beaten ss a bunch of times. out of hell he's been utterly dismissed and thought inconsequential by thanos and warlock and has showed serious deference to odin.
being in hell does not ensure he wins, but i agree mephisto wins there. i'm just not sure ds could do anything to ensure mephisto STAYED down. i do NOT think it would be easy though. outside hell, ds would treat him as inconsequentially as those others have.
and mephisto is NOT the biblical satan. there was a theory put forward once in hellstorm or strange or one of those occult titles that claimed each of the 'great demons' are actually only a portion of a GREATER demon, the 'true' great evil. satannish, mephisto, asmodeus and some others were all said to have been one being once and it was said that someday (if they stop hating each other) they may once again become said being.
not sure if marvel will ever follow that up, but it was a sort of cool idea. 🙂