how the hell did you do that?
The secret is to kill the demon first, since the only thing that can hurt him is bullets to the heart, and Wulf will help refill guns when he drops them.
The hard thing is managing to stay alive long enough, and to have a good spot from which to fire in relative safety. After some size increases, you will have to go up the columns to be able to hit, and you may want to use the zoom feature too. I was lucky in my final try because Wulf picked up the rocket launcher, and just started ripping Beliar a new one.
Once Beliar falls, you have a save chance, and then it's just you and Wulf. Bullet Time helps a lot in that fight, especially since your health won't be full.
06/01/2012: Magic: the Gathering - Duels of the Planeswalkers 2012
09/01/2012: Portal 2
18/01/2012: Gabriel Knight: Sins of the Fathers
An excellent game from the Sierra Adventure Days, with few puzzles needing hints. I hated the "quick reaction" sequences though.