Know that this first part is mostly a battle made up of you blocking and then countering with Fury whenever you have your Dark Gift energy meter filled high enough. Faustus takes little damage from normal hits, even from the longsword.
After three fury hits, he will run away saying that he has a gift for you. Follow him through the tunnel and into the big room. As you approach him, he'll leap atop one of the four furnaces that sit in the middle of each of the four walls. Run over to the base of the one he's standing on and pull the lever to douse him in flames.
He'll fall to the center of the room in pain, but then do the same thing again. However this time he runs as you approach the furnaces. You must be quick. Don't pull any levers until you've timed it so that you're about to pull it just as he has reached the furnace; otherwise he'll just run as you flick the switch.
After this second hit, he'll go back to an easy version where he throws better aimed shots at you; although if you pull the switch in time you'll be invincible to any hits you may take. At times, mist will come out and allow you to sneak up on him while he just stands there. Try using it to your advantage.
In the final phase of his attack, Faustus will climb to the very top of the room and run around the rafters trying to sneak up on you. At the start of this phase, all the mist in the room will dissipate. You need to either find a good spot where you can watch him at all times, or listen closely. When you hear his wispy breath, it means he's dropped to the floor and is attempting to kick you (he performs a VERY long and powerful kick, lined with red energy). All you have to do is run when you hear this sound. If he does do the kick, he'll miss by a longshot and end up skidding along the ground. He'll be stunned for a second, so run up and get a good three hit combo on him.
After four combos he should die.