I have done it 10 times using the cheese spot under the walkway. I prefer a Titan with Sweet Business and Actum War Rig. It will stun him and he will stay in the same place the entire phase. Shoot slower when you are about to run out of ammo to give Actum War Rig time to refill magazine. Most of the time the Tormentors get in the way so you have kill them too. I also have an explosive round scout to shoot in his direction if he walks away during the final phase. It will get his attention or shoot through the circular flooring when a psion gets in the way. I used Osteo Striga once. Reload takes too long and he wanders.