The witch that has been a thorn in your side finally joins the fight. During the fight she will tend to back away from you and cast magic from afar. She also has lower health than what you might expect from a boss, but she makes up for this with her energy shield. At each quarter interval of health she will deploy a shield that makes her invincible. Once it’s up she also summons minions to keep you busy. The minions aren't very tough and you should be able to dispatch them with ease. But be careful because they do pack a punch.
Once you take care of them Maghda will show herself again and you can continue chipping away her health. Her basic attacks won't damage you a whole lot so you shouldn't worry too much about dodging around or avoiding her attacks. Simply attack her with your most powerful moves and before long she will be down.
Zoltun is quite the powerful mage with some pretty big backup. Zoltun animates two large magical golems you will have to down before taking on Kulle himself. The golems do hit hard but they are so slow that you should be able to stay at a safe distance while attacking them. Once you take them out you can shift your focus back to Zoltun. Since he is a mage he has a variety of tricks he will use at a distance. His fireball does moderate damage, but often times he will only shoot one at you, so it shouldn't be much of a worry. The tornados do less damage but you should still step out of their way.
Finally he can summon a bubble of slow time around you. This does no damage and you can simply walk out of it to return to normal speed. Sometimes this will slow you down enough for a piece of the roof to fall and damage you, however. Similar to other mages, he is very weak physically and has a fairly low health pool. It shouldn't take long to knock through all his health and send him back to the land of the dead.