This is a very interesting find! Well, according to a friend of mine this is old news and has been known for years, but currently I can't find anything about this outside the new video and posts related to it. Anyhow, in case you didn't know, it turns out that pouring Forest Water onto Kalle Demos' "carpel" will instantly defeat the boss.
Oddly enough, this does make quite some sense and it's details like this that make Zelda games so great. Years later you can still find out some pretty curious things. Of course with the 20 minute time limit (30 in the Wind Waker HD) you either have to beat the dungeon very quickly or get out of your way to collect the Forest Water right before the boss. But I still will try this on my next run through the game.
(Thanks to angelday for the tip!)