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Chapter 6: Embrace the Dark- Nohr Route
Interacting with the dragon vein turns the river tiles into plains tiles allowing you to defeat Ryoma with someone other than Xander or run away from Ryoma. It isn't necessary to defeat Ryoma.
Only Corrin and Felicia/Jakob are able to gain experience in this chapter but don't be afraid to use your other units if you have to. Use the dragon vein if you want to let Xander run from Ryoma or let someone else kill Ryoma. It isn't required to kill Ryoma as the objective is to kill 4 out of the 5 enemies. Leo can be an effective tank, just don't let Takumi or Yukimura attack Camilla.