Veterans who played "Skyward Sword" would recognize this area where the giant black monster "The Imprisoned" lives. Not really an image of a monster in chains. Also, this is where the mysterious old woman lives. The only spoiler is the old woman is one of the warriors in the Zelda cast.
Traitor amongst us?!
After taking the nearest base which is the north west near the middle, many of the soldier leaders starts betraying your allies. Start taking down Turncoat Soldiers as they randomly appear on the map. Make sure you get the rest of the middle bases as they were important later on. Wait until Fi tells you that there's a leader that's controlling the soldiers. It's random and could be in one of the ally bases. Take one down and the rest should go away.
Consider this as Zelda's version of Godzilla. The idea is to attack at the toes of the monster. Both has three nails at the front and one on the back. The stomp impact might hurt you so be ready to attack in full health. Be sure to use both special attack and spirit attack to cut him down more easily as he will go to one of the bases. Once that happens, you have to use the Groosenator to knock it down. Actually, it's a wooden catapult with a bomb at the ready. And the name Groosenator is from Groose. He has red hair and use to be a bully until coming to this place has he got a change of heart and decide to help you out. He's not in this game so play "Skyward Sword" to see his appearance. It will take two to three times to completely knock him down.
Although it will take a while because of all the monsters and the leaders and the bases, especially the north side of the map, Girahim is located at the north west corner of the map. He'll continue to summon moblins so go to his lair and knock him out completely in a heated battle. A cutscene will see him get knocked into the "Gate of Souls" and you using the slice down technique to close the gate. This should mark the end of the split up depending on whether you done "Ocarina of Time" and "Twilight Princess" earlier or later.
Go to the very east of the map. Use the boomerang to get your reward. Do it after you reach one thousand kills.