If this gets closed then nobody'll mind me posting these?
Mephisto can also take your soul if you willingly "offer" it to him in a dream, even if you don't believe it to be real.
Here he makes a pact with Marlo Jones for her soul in her dream.
http://img58.imageshack.us/my.php?image=scanmarvel67no.jpg
http://img58.imageshack.us/my.php?image=scanmarvel74qq.jpg
Tricks Thor, Sif, and Balder.
http://img58.imageshack.us/my.php?image=scanmarvel56rt.jpg
Challenges everyone in hell (or just an immeasurable number of souls) to rise against him. There are no takers.
http://img115.imageshack.us/my.php?image=mephisto018zx.jpg
Chucks Blackheart god knows how far into hell.
http://img115.imageshack.us/my.php?image=mephisto028ev.jpg
Eating souls (and JESUS! Look how huge he can get!)
http://img122.imageshack.us/my.php?image=mephisto037zl.jpg
Below are excerpts from a comic that is written by Stan Lee:
Mephisto battles Silver Surfer and wins relatively easy.
http://img402.imageshack.us/img402/9768/mephistobattlessurferod9.jpg
http://img402.imageshack.us/img402/3669/mephistobattlessurfer1kl8.jpg
http://img162.imageshack.us/img162/2109/mephistobattlessurfer2ht9.jpg
http://img403.imageshack.us/img403/1241/mephistobattlessurfer3dc4.jpg
Mephisto battles Galactus where he actually starts overpowering him.
http://img403.imageshack.us/img403/7307/mephistobattlesgalactusel3.jpg
http://img403.imageshack.us/img403/8203/mephistobattlesgalactuseg2.jpg
http://img19.imageshack.us/img19/4581/mephistobattlesgalactusan2.jpg
http://img243.imageshack.us/img243/2618/mephistobattlesgalactusjy6.jpg
http://img403.imageshack.us/img403/5421/mephistobattlesgalactusdw9.jpg
http://img403.imageshack.us/img403/11/mephistobattlesgalactusid1.jpg
http://img228.imageshack.us/img228/2592/mephistobattlesgalactusja9.jpg
http://img169.imageshack.us/img169/695/mephistobattlesgalactushq3.jpg
http://img209.imageshack.us/img209/8002/mephistobattlesgalactusyp9.jpg
Now the rest are from different writers:
Mephisto battles Thor.
http://img187.imageshack.us/img187/4985/mephistothornu1.jpg
http://img152.imageshack.us/img152/2517/mephistothor1qx0.jpg
http://img152.imageshack.us/img152/4678/mephistothor2so2.jpg
http://img243.imageshack.us/img243/7043/mephistothor3ni6.jpg
http://img243.imageshack.us/img243/3503/mephistothor4px1.jpg
http://img243.imageshack.us/img243/3350/mephistothor5rc9.jpg
http://img95.imageshack.us/img95/1830/mephistothor6ng4.jpg
http://img82.imageshack.us/img82/4627/mephisto7oy6.jpg
http://img82.imageshack.us/img82/1181/mephistothor8xc7.jpg
http://img150.imageshack.us/img150/6671/mephistothor9az3.jpg
http://img150.imageshack.us/img150/3573/mephistothor10jb1.jpg
http://img401.imageshack.us/img401/4817/mephistothor11on3.jpg
http://img401.imageshack.us/img401/5970/mephistothor12ft1.jpg
Mephisto versus Wolverine:
http://img169.imageshack.us/img169/5703/mephistowolverineqm9.jpg
My own contribution (I'll add more later, this was just a quick run through some comics):
Mephisto traps the Defenders (they only get out when Satan frees them)
http://img471.imageshack.us/my.php?image=defenders09919cy7.jpg
http://img471.imageshack.us/my.php?image=defenders09920lu7.jpg
Mephisto explains to Surfer that he has always feigned defeat
http://img84.imageshack.us/my.php?image=defenders10025pd9.jpg
Mephisto kills (yes that's right KILLS) Surfer, Clea, and Valkyre with ease
http://img411.imageshack.us/my.php?image=defenders10029jy7.jpg
http://img395.imageshack.us/my.php?image=defenders10030pb8.jpg
Mephisto casually redirects the attacks of Strange and Doom
http://img395.imageshack.us/my.php?image=drsstrangeanddoom71tt7.jpg
Doom has had years, if not decades, of one sided prep to defeat Mephisto, this is the end result
http://img412.imageshack.us/my.php?image=drsstrangeanddoom73lt7.jpg
http://img411.imageshack.us/my.php?image=drsstrangeanddoom74lh3.jpg
Once again, Strange cannot escape a prison conjured by Mephisto, his only hope is to escape to Aggy's dimension via the Eye
http://img412.imageshack.us/my.php?image=drstrangesorcerersupre0ac8.jpg
http://img412.imageshack.us/my.php?image=drstrangesorcerersupre0bg2.jpg
http://img411.imageshack.us/my.php?image=drstrangesorcerersupre0px4.jpg
http://img411.imageshack.us/my.php?image=drstrangesorcerersupre0ie5.jpg
Example of how large he can grow (the first one is just to give you an idea of how big Satannish is to begin with)
http://img412.imageshack.us/my.php?image=drstrangesorcerersupre3br8.jpg
http://img213.imageshack.us/my.php?image=drstrangesorcerersupre3my6.jpg
Strange's views on how he compares to Mephisto
http://img412.imageshack.us/my.php?image=drstrangesorcerersupre3jv2.jpg
Mephisto vs. Thing
http://img213.imageshack.us/my.php?image=mvs116ns4.jpg
Laughs at Sue's best attacks (take THAT Exitar!)
http://img443.imageshack.us/my.php?image=mvs121my7.jpg
Reed, with as much prep time as he wants, realizes the futility of fighting Mephisto, even with Sue's life in the balance
http://img134.imageshack.us/my.php?image=mvs2pg03lm5.jpg
Owns Iceman...guess it's also good reaction time against Cyc
http://img243.imageshack.us/my.php?image=mvs2pg15ro4.jpg
Converts Psylocke's psi-probe into deafening sound (telepathy into sound, not bad) from his dimension
http://img443.imageshack.us/my.php?image=mvs308fy9.jpg
Doesn't budge when Rogue let's loose on him
http://img243.imageshack.us/my.php?image=mvs311du1.jpg
Can steal souls with a touch (and isn't affected by Rogue's powers)
http://img530.imageshack.us/my.php?image=mvs322mj0.jpg
Hela flees from Mephisto to free Thor's soul is unable to remove the mystic mylar he wrapped Thor's soul in
http://img443.imageshack.us/my.php?image=mvs413iq6.jpg
Just as a note, Mephisto's powers were weakening at an alarming rate by the time he encounters Hela and the Avengers
Owns She Hulk, Iron Man, Photon (take THAT Zeus!), Dr. Druid, Hawkeye, Wonderman, and Tigra, all the while searching the infinite number of dimensions for Hela and Thor
http://img135.imageshack.us/my.php?image=mvs414fq7.jpg
http://img443.imageshack.us/my.php?image=mvs416vv5.jpg
Sons Cap, then proceeds to own him and Black Knight
http://img300.imageshack.us/my.php?image=mvs417sd0.jpg
Owns Hela, and she doesn't even realize it...plus it's just an awesome scene
http://img300.imageshack.us/my.php?image=mvs421ey2.jpg
http://img353.imageshack.us/my.php?image=mvs424cn2.jpg
Mephisto vs Ulik:
http://img411.imageshack.us/img411/2971/mephistovsulik1ag6.jpg
http://img149.imageshack.us/img149/6659/mephistovsulik2tf8.jpg
http://img182.imageshack.us/img182/5518/mephistovsulik3oc5.jpg
http://img226.imageshack.us/img226/2738/mephistovsulikvo0.jpg
(Most scans ripped from HC)