Tank the Boss in the middle of the room. Make sure you have a firebomb under it so when it goes invisible and moves it will be shown again. Beside that, physical damage is very good against it, as for mages just the normal wave rotation. Also you should have the boss diagonal to the EK since the beam does mana drain a lot and also life leech (if I don't remember it wrong).
With having the boss in the middle of the room you will get most of the summons on the EK without having to run around.