After defeating the impostor, you receive a strange prophecy before going toward the Bottomless Canyon.
Preparing Before the Battle
When you are setting up your units, be sure to include Azura, and put her in the middle of the left side of the map.
Note that Sky Knights can fly over the thick forest areas, so they can move much more easily than your other units. Be sure to include your Sky Knights in this battle.
There is also a chest on the right side of the map that one of your Ninjas can open, but the door to the chest is guarded by the boss unit.
Rout the Enemy
As you can see in the map, there are several Dragon Veins, each of which will turn part of the ground into spikes. Any unit (friend or foe) that is on one of these spike tiles at the beginning of its turn will lose 10% of HP.
Furthermore, if you step on certain tiles on the map, hidden spikes can be activated.
Soon after the start of the battle, Takumi will appear, but something is wrong with him. You have to have Azura talk to him (she doesn't need to sing). When you do, he will go back to normal and join you. If you are unable to have Azura talk to him, you will have to kill him.
Have one of your Ninjas pair up with a Sky Knight, and have them make their way to the chest on the right side of the map. With a Sky Knight to transport your Ninja, there will be no need to unlock the door that Kotaro is guarding. Once you reach the chest, have the Ninja/Sky Knight separate, then have the Ninja open the chest. It contains Rescue.