Before you go to Mephisto's realm in Act 2, equip Magneto (DLC character) in your party. Once you enter Mephisto's realm, when Xavier talks to you, Magneto will respond to him and the achievement will unlock.
I am also led to believe the DLC characters are no longer available for download. However they are included in the Gold edition of this game.
Thanks to everyone in the comments for bringing this to my attention