After reaching the Bottomless Pit, you are attacked before you even hit the ground.
Before the Battle
Be sure to use all of your royal units in this battle, because you will have to activate a lot of Dragon Veins before you can reach the boss unit.
Defeat the Boss
To finish this battle, you have to reach the room with the boss unit. Before you can do that, you have to activate all of the Dragon Veins. To get to the Dragon Veins, you have to depart from the island where you started, and you can do that by using the Sigils on that island. Each Sigil leads to a different part of the map, so send one royal unit through each one. Or you can concentrate your royals in one area of the map that slowly make their way to all of the Dragon Veins.
Once all of the Dragon Veins have been activated, the statue in the southern part of the map will disappear, revealing a Sigil that leads to the boss unit's room. Once you are ready, have everyone warp back to the first island (there is a Sigil on the west side and the east side that take you back) and then crowd around the center sigil in preparation for the assault on the boss unit's room.
Kill the hero units that surround the boss unit, then attack the boss unit herself. She wields a Rabbit Spirit and a Sun Festal. Once she is defeated, the battle ends.