Twinmold is a boss from the Legend of Zelda series making it's only appearance in Majora's Mask.
The funny thing about the fight is that Twinmold is about 1000x bigger than you are... So you have to stand there shooting at it with arrows.
Unless of course you remember about the Giant's Mask. It's a tease of an item though. The only place you can use it is against Twinmold.
It turns you big, and now Twinmold is easy to hit with a sword. Kinda of a shame, makes the battle too easy.