After killing Fabius the guy on the ritual table will wake up and break free of the binds. You need to kill him. Luckily for you the combustion ability will be available. Charge up a shot and toss it at the guy.
Now run around the area and press 1 to use your hands. Flip over the crosses so they don't make you weak. The guy is fast though and he'll catch up to you and turn them right side up again.
To be able to hit him with the combustion skill, run around the room and turn around 2-3 crosses, then turn around and shoot him. He shouldn't be able to keep up with you if you don't take breaks in between turning crosses.
Keep on repeating this pattern and he'll eventually burn up and die.