How would you defeat an Iron Lich and its Fiend Cohort?

[Mehangel]

I am currently at a loss as our dm is sending us to defeat a Level 15 Iron Lich (Converted from Iron Kingdoms, I think) who also has a cohort which is probably a Half-fiend, Succubus, or Erinyes, which I am not entirely sure... Anyways, the adventurers going out to them down are all level 15 and include an Elf Technomancer/Psychic Warrior, a Human Zen-Archer Monk/Wizard, and myself a Xeph Nimble-Gifted-Blade (10)/Dark Tempest (5). Each of us also just recently gained leadership, and thus have cohorts but I have no information regarding the cohorts of the other two players and I have little intention of sending mine out into the battle field, though if i do, the cohort is a Noral Expert (3)/Tactician (9).

So I suppose my question is how would you go about taking out the Iron Lich and its cohort? Should we focus on taking out the cohort first or the Lich? Should we divide our forces between the two? Should we focus on destroying its furnace (which it must carry with it to cast spells)? What information should we attempt to gather before we engage?

I understand that this isn't alot of information, but I was curious as to your thoughts on the encounter...

EDIT: The cohort of the Zen-Archer Monk/Wizard will be joining us in the fight, she is a Witch (5)/Exploiter Wizard (5).