Most would agree that the Iron Passage is one of the toughest parts of not only the new DLC, but the entire game. Simply getting to the boss is extremely tough, and if you manage that, you have one of the toughest boss fights in the game on your hands, even with the NPC or player summons helping.
In this video I attempt to show you how I beat it. It is not the only way, nor necessarily the best way, but it worked for me, so I hope it helps you guys. I know I missed a lot of items here, but that was intentional, as I wanted to get straight to the boss and through the zone.