After returning to Nohr with Azura, you take a ship to Hoshido but must defend the war chest from a hidden enemy.
Defeat the Boss
As you will notice if you view the map before the battle, there is no boss unit to be seen. When you start the battle, you learn that one of your ally units is secretly a ninja who is stealing from the war chest throughout the battle. The war chest contains 10000 G, but you will lose 300 G each turn until the battle is over. You will win whatever amount of gold remains in the chest when you finish the battle.
One of the enemies here will drop a Master Seal when you defeat it, and another will drop a Heart Seal.
To find the boss, you will have to talk to all of the green Nohrian Lancers. The last one that you talk to will be the boss. Anyone can talk to the lancers, not just Corrin, so split everyone up to talk to the four lancers as soon as possible.
Be aware that these lancers never move or attack.
The Adventurer enemies on the east side of the map can cast Freeze and Enfeeble on your units, so keep an eye on their spell range.
After you have talked to all four ally units, the boss unit will be revealed. He is an Adventurer with a bow. He never seems to move (this might only happen in Normal difficulty) so get close to him and he won't be able to attack you.
After the Battle
Afterward, you will have to decide whether to spare Shura's life or execute him. If you do not kill him, he will join you as a playable unit.