When you enter the room, they show him to you, once, (he's green) and then he disappears and begins shooting fireballs at you from random locations in the room. You basically need to stab around wildly until you hit him; you'll know that you've hit him because he'll appear for a second (still green). Do this several times (I believe you have to hit him with your sword 3 times total) and then he will appear brown (instead of green); shoot him once with your silver arrow (gotten in level 9) and he's dead.

how can i beat level 9 it is very hard