Magnus is fairly simple to defeat. When he is standing in the middle of the giant fountain that is surrounded by statues, run and hide behind the closest one. He will try to burn you, but will hit the statue. Jump out and shoot him with Telekinesis very quickly. Once he falls into the water, run behind the next statue until he tries to burn you again and repeat the process until he leaves. It should only take three times. In the next room you will see giant statues holding scythes. Magnus cannot burn you anymore, but he will charge into you. Do not bother hitting him. It does not do any good. Get in front of one of these statues and get him to charge you. When he does, dodge him and he will run into the scythe, breaking it. Once it breaks, you will see a glowing telekinetic switch. Jump onto one of the boulders or platforms so Magnus cannot get you, and shoot the switch. The statue will open its arms. Do that to all of the statues and the gigantic statue in the middle of the room will be put back together. Get Magnus to charge into the base of the weapon that the statue is holding the same way as before. When it breaks, make him charge into the base until the statue falls and crushes him. Just stand next to it and shoot Magnus with Telekinesis. That should make him charge you.