After battling through Notre Sagesse and gaining more allies, you reach the Sevenfold Sanctuary in search of the Rainbow Sage.
This is another map that will look familiar if you have played Birthright or Conquest.
You will want to split up your units, sending some of them up the stairs on the left and some of them up the stairs on the right.
However, you should leave Corrin in the first room, because after a few turns, Nyx will show up, and you can get her to join by having Corrin talk to her. After that, you should send Corrin up the stairs on the right, where she can talk to Shura to get him to join. Be sure not to attack him or get in his attack range before then.
Once you recruit Shura, he can open the chests for you with his Locktouch skill, if you don't already have any Locktouch units with you.
Once you are ready, head up to the second-to-last room, where your units will reunite, and fight your way to the top to defeat the unit that is standing on the seize tile.