If you mean the Boss Key, have Zelda (while controlling a Torch Phantom) grab the key and trace a path to the lock, since the electric field keeps Link from grabbing it. You'll need to fend off three Key Masters after she takes it. You can temporarily defeat them, but they'll return soon afterward.
Next, it's a fight against a Geozard Chief. Double-team him, and you'll get the map in the next room.
BTW, a similar puzzle occurs on floor 23.