I've decided who it's going to be. She'll be an "imposter Zelda," Not exactly an imposter of NeoFirst's Zelda, though, but an imposter of Queen Zelda I, who, as chronicled in the Plot Tracker topic, defeated Grymwald on Mt. Frostburn and founded the Kingdom of Hyrule a long time prior.

The imposter will claim to be of royal lineage, and seeks the same Triforce of Power which lies at the summit, just as it did when Zelda I claimed it so long past.

She'll resemble First Quest's Zelda - long red hair, and unnecessarily shiny plate armour. As you know, she'll be armed with the hammer. When you enter the battle room, you'll see her dealing the final stroke to Fire Volvagia (the twin of Ice Volvagia, the other L2 Miniboss). Her attacks will involve using the Quake Hammer to stun you and drop stalagtites from the ceiling.

To beat her, you'll need to use bombs to do X amount of damage to her, which will trigger a cutscene where Trinexx* breaks through the north wall, swallows her, then breaks through the south wall - thus creating the two doorways marked on the map at 3A. Of course, the northward doorway is blocked with lava.

*Oh, by the way: Ice Volvagia and Fire Volvagia are actually just the two "elemental heads" of Trinexx. As you probably forgot, destroying both heads causes the central head to change into "snake form", which is how it will appear in the cutscene. All of this will either be explained in-game, or just left as Yet Another In-joke for LttP fans to enjoy miss entirely. (Who knows, maybe in the NeoSecond version of this dungeon you'll be allowed to fight Trinexx proper.)