Versus Mephisto its actually a TWO (2) part fight.
First off: You'll be fighting invunrable Mephisto (life gauge is silver instead of red), keep beating on him until he pulls out his blazing sword. When he pulls out his Red Sword, fight it off of him (hit the (X) on the X-Box) pick it up, and hit the STATUES. Don't bother using it on Mephisto, as it *does* hurt him some, the primary objective is to release the soul of the X-Man you need to save. And besides, you'll want to save your health & energy for part two.
Part the Second: It'll be 4 on 2 now; your team (or whoever survived pt.1) versus Mephisto AND the resurrected X-Man (Jean in your case). Now is when you just go nuts, use your X-Treme moves and the kitchen sink. For this one, i prefer focusing in on Jean, b/c she'll end up helping you against Mephisto. In my case, she sacrificed herself to suck him into a black hole..
That's what I remember; let me know if it doesn't make sense or whatever. I just got to Dr. Doom in his castle, and I can't make it past his "Fearsome Four".