After defeating the traitor, you will face off against Anankos.
Before the Battle
You won't be able to go back to your castle or save after this battle is over, so if you aren't ready, choose Exit so you can return to your castle to prepare.
Defeat the Boss
You can use the many Dragon Veins in this map to turn each one into a single healtile. This will also prevent reinforcements from coming out in certain places.
Use the Omega Yato on Anankos to deal a lot of damage in one turn. Your other units will not be able to deal as much damage, but they can chip away at his HP so Corrin can deal the final blow.