Vitreous

Vitreous (ゲルドーガ, Gerudōga; lit. "Geldogger"), the boss of Misery Mire and captor of the sixth maiden, is a huge, gigantic eyeball surrounded by smaller eyeballs, immersed in a green liquid.

Strategy
The main eyeball will shoot lightning at Link, similar to one of Agahnim's attacks, while the smaller eyeballs will attempt to swarm him. To avoid this, stand in one of the corners of the room and use the spin attack. Once the smaller eyeballs have been taken out, the main eyeball will attempt to charge at Link. This giant, massive eyeball can be defeated with sword strikes, but takes more damage from arrows. Defeating Vitreous earns Link a Heart Container and the sixth imprisoned maiden.

Alternatively, Link can also use the Magic Cape to dash right into Vitreous and slash him until he is defeated.

Trivia
Vitreous is called "Geldogger" in Japanese. The suffix -dogger, in addition to its eyeball appearance, connects it to Digdogger, a recurring boss in the Zelda series. The prefix gel- is probably used due to the fact Vitreous is immersed in a green liquid.