You'll need a Purify spell from Duncraig if you want to damage the Flashlight, though. So, before you fight, backtrack from the Cave of Mysticism, and take a right at the save point. There should be a small crack in the wall, but you have to stand on a certain spot exactly to analyze it. Just keep trying. Once you do, you should enter Duncraig.

Beware the exploding barrels and green FLASH monsters. They're dressed exactly like the Flash, but green, and attack with giant oversized atoms. Make your way to the tentacle monster in the back. Then you'll get into a battle with Fleshlight, and once he's finished, listen to his story, and he'll give you the Purify spell so that you can attack the Flashlight, his mortal enemy. Purify comes on a rubber band that works only when equipped on your...

...more manly appendages.

Or attack the shadows. It isn't hard, you just need to make them as small as possible before you attack, since their HP changes based on size.