Titannus is straightforward but tough to kill by any standard. The best way to kill him is to use someone with a heavy ranged attack.. Captain America's Shield through or Thor's Hammer Throw work nicely.
Next is to try to get him blocked into an area where you can stand some distance away and attack while he cannot get at you. I've had this happen in a couple of different doorways where he was too big to get through but I could stand a couple yards away and throw power attacks at him.
One of the possibilities is a glitch where you can get him trapped in one of the small single rooms with the forcefield and demons behind it. He'll follow you in, then when you leave he can become trapped. However, this can be a little dangerous too, because it is possible for you to get stuck behind him and have trouble getting to the main hall.