Did you hit him once yet? You're trying to get the ray of light to the gun so it'll git the monster. For the first hit, you just pull the lever. For the next hits, you have to figure out where to point the light beam and how to direct a path of light to the gun.

To do this, you have to use the ice mirrors by the dragon fountains (available after the first hit). All of the things in the middle of the map are placeholders for mirrors, and they can either hold a mirror diagonally (X) or sideways (+). Depending on how you fire the light and how you place the mirrors will affect its reflection. The angle you fire the light at the mirror is the same it'll fire off.
         
        .......\
.......]mirror (placed vertically) will fire in a diagonal                                           
light/                                                               

light------>]mirror will fire straight back 

.............|
light-----/mirror (diagonally placed) will fire at right angle


But don't get caught out in the middle of nowhere when he fires his icy breath--take cover either by an icicle, the lever, or a diagonally placed mirror. Or you'll die.

And if you want to see the actual path, the Oracle will tell you if you use a doll. =)