Mephisto is a hel lord. He has battled Galactus to a stand still Galactus just tried eating his dimension and that forced Mephisto to let him go.
He has gone up against Thor and Surfer but the other poster was right, he was in no danger. Thor couldn't really do anything and Surfer i think just dreamt that he kill him. He just wants their souls, since they are pure that's where his trouble begins. Oddly enough only two people i think have actually really threatened him. Magik w/c is understandable cause of the soul sword and limbo power and the Hulk ( pis) for me. He just strangled him...
Etrigan seems to be on a much lower level (herald level) from his showings but i dont really follow him.. so i could be wrong.
Right. When Mephisto and Galactus clashed, it caused planets to start blowing up. Etrigan hasn't shown anything remotely close to that output. Mephisto would have somebody like Etrigan working for him.
He did walk the dog on Sentinels of Magic, in which MM/Fate said he could tap into limitless power. Give him Lucifer's trident and Stonehenge power and I can see Etrigan competing with Meph.