You will initially attack Mephisto just as you would any sub-boss. Eventually he'll start using his sword. When this happens get close in front of him and press the 'X' button. You'll see a flashing 'Y' button at the top of the screen. Just start button mashing the Y until you wrest his sword away from him. Once you have it, run over and smash one of the statues at the points of the pentagram. Repeat until you have smashed them all.
After the 5th statue is destroyed either Jean Grey or Nightcrawler will appear (whichever one you didn't save n the prior level). They will be under Mephisto's control, so you must fight them.
It is probably simpler to avoid Jean/Kurt & just keep attacking Mephisto until his health bar gets down to the last 15% or so. Once there a single hit or two on Jean/Kurt will also kill Mephisto. Alternately, you can attack the minion first and their health bar will bring down Mephisto at the same time.