Sorry, I don't think I was clear. What I mean is you can ignore that enemy down the stairs entirely for right now. You don't need to beat it to finish this area, and later on in the game, you'll be able to come back here. What I'm guessing you need to do is on this screen:
Coming from below (use the fire rod to get rid of the plants and snow drifts) will let you hit the switch, which turns the northern block into a keyhole. Use the key there to get the boss key.
While I have a lot of issues with the quest, I do still think it's quite beautiful, so I'm glad to see people appreciating that aspect of that.