Magnus is pretty easy to defeat. When he is standing in the center of the large 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 quickly shoot him with Telekinesis. 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 large 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, evade 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 large 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.
-Whilee fighting Mangus, it is much easier if you just grab him and throw him into the giant statues holding scythes. Even if you miss with the throw, he can still bump into them when he is getting up, and they will still break.
-When the battle starts, stand directly behind one of the statues. Beware as his Immolate attack can kill you with one hit. After he destroys the statue, use the Telekinesis gift to knock him into the water. You will need to do this about four times. Enter the next area with the Telekinesis still equipped. In this area, you will see four statues glowing red. Stand in front of one and shoot Magnus with the Telekinesis. When he charges at you, jump over him. A switch will appear at the feet of the statue. Use your Telekinesis to operate it before Magnus recovers. Do the same for the other four, and the statue in the middle will rebuild itself. Next, use the Statue technique on the large one; you may need to do this more than once. This should finish him off.