Kalle Demos, the boss of the Forest Haven dungeon, can be defeated in a single hit. When you use the Boomerang to cut the boss's vines and knock the plant onto the ground, a small plant inside of it will be revealed. If you pour Forest Water onto this plant (which is the boss), the boss will be instantly defeated.
To upgrade your Arrow, Bomb, Magic, and Rupee-carrying capacity, you must visit the eight Great Fairies hidden throughout the Great Sea (certain tools are required).