During the scene, you’ll receive the Shadow Yato. In this map, you can deploy 16 units. However, some of them will be left alone in the rooms on both sides of the main corridor. You can however, pair these units, especially your self-sustaining or powerful units. Each of the three rooms has healers so make sure to take out these healers first. The stairs in this room will allow you to bypass some enemies in the middle and take you to the six stairs near the throne room. However, you can opt to move all your units in the main corridor instead.
There are no reinforcements for the entire mission so this will be surprisingly easy for you. Garon will not move from his position so you can gather your forces around him and beat him down. He has high HP while bows and tomes barely damage him. Shadow Yato is very strong against him so you have to use your avatar to defeat him.