I think we are speaking about the same stairs. I've tanked him in the corner to the right side (facing down the stairs) of the foot of those stairs, right at that lil corner. My wife grabbed the adds right at the doorway to his chamber. Worked first try, so I didn't really bother with this anymore. Maybe you need a bit more distance between your tank and add positioning.
The boss hits like a girl (afaik your tank is sitting at 35k+ unbuffed health too), so am sure you can move him around a bit while in combat.
It kinda looked like this:
Code:X Boss spawn spot
X Doorway (grab adds here)
X Boss fight spot
X Stairs