Taking an Epic level Psion? That rogue that seems not to have a "niche" might be the best hope you have, especially if you can get her some severe poisons or a spell storing short sword.
She will likely have a high dex, but make sure to buff it as high as can be reached before Combat. If you are going the offensive, which is essential, you are going to want to catch him flatfooted. If you can get cleric stored harms or another very damaging spell in the spell storing swords, combined with Sneak Attack damage will weaken him considerable, especially seeing how even if he is a twinked epic psi, his hit points aren't going to be great.
If you go the posion route, try to pick one that attacks con or Charisma (it will be low, and if you drop it to zero it is catatonic.)
Another tactic is go reverse adventurer on him, especially if you can get your hands on some Golems. Find a defensible location. Block it against teleports and Gates. (forbidence, etc.) Then stay in the central chamber, which is warreded against scrying and mental attacks.
Out from that chamber, have your clerics, wizard, etc,and especially rogue, fill the way to you with as many traps (mystic, normal, and otherwise) as you can. It is hard to read a trap or a Golemns mind, making hard for him to spot things.