Mephisto is a great champion and an AQ beast. Use sp3 when opponent has more than 2 bars of power, he drains 2 bars and gets his aura for the rest of the fight. Gains one charge per matchup which can heal you and allow you to rank sp3’s as well as unblockable attacks without dying. I was hesitant to rank 4 my 5* but I am glad I did it. Nice pull. Also you can use sp1 so that defenders can not activate buffs (btw they start the match with this active for like 10-15 seconds).
I run my 6* Mephisto with Void, Omega Red, and some others or, if you're building a team around him, Symbiote Supreme, Ghost Rider, Champion, and ... (I would say Doctor Strange but he's not even worth having around for synergy)... can't remember his other good synergy.
He's more than decent unawakened, you can always spam the sp2 to get aura's. Don't forget he disables buffs at the start of the fight, with some nodes this is a gamechanger. He can also totally wreck Iceman (disables ice armor + immune to cold attacks). And as mentioned before, he can power control once every fight.
Comments
Don't r2 him, sure, but he doesn't suck. Compared to Domino, SS, and others, sure, but compared to Magneto, Doctor Strange, Sentry....etc, he's a god.