Get the boss at the top near the purple sphere, summon Summon Livor Mortis (with modifier increasing threat) and use again the spell multiple time so that the boss attack your summon and stay on the top on the map near the purple sphere.

The purple sphere will do lots of damage to the boss and will also increase damage you do to the boss after emitting it's way of energy. Be carefull to don't be hurt by the wave so stay away of the purple sphere AOE.