After defeating Hinoka, you invade the Hoshidan capital and find Ryoma in the throne room.

Defeat the Boss

Corrin and Ryoma will be sealed in the throne room for a duel. Ryoma won't do anything for 25 turns, so you could just have Corrin beat him up until he is defeated, taking Vulneraries and Concoctions from the Convoy if needed.

If you want your units to get more experience, however, you should send them to the left and/or right to fight the enemies there.

If you defeat Saizo or Kagero, the path to the throne room will open, allowing you to send your units to gang up on Ryoma. There is also a Dragon Vein near the entrance to the throne room that will turn the Healtiles into Avoid-penalty tiles, reducing Ryoma's ability to avoid attacks.