Sure, like I said I can't get in game this evening so details might be vague.
I also tried Special, that wasn't working either btw.
So when the fight starts there is text along the lines of Boss is immune to all damage but physical, and he gets a buff (blue iirc, but same icon whatever the damage type which is annoying) if you mouse over it you can see what he's vulnerable to at the moment.
Around the main building there are 4 little roofed areas, one either side of where named 1 and 2 spawned. Each one contains a lantern which is clickable, and you can 'harness its power'. One is physical, one nox, one arcane, one elemental.
You need to watch the chat box to see his vulnerability change, and mouse over the buff to see which lantern to click.
I think that's right, someone will correct me if not I'm sure.
Hope that helps!