After the intro scene, you find yourself in a battle.

Rout the Enemy (All Difficulty Levels)

First, choose Corrin (your unit), then choose the enemy unit, and choose attack.

After the enemies and your allies make their moves, a Lancer appears next to you. Have Takumi attack the Lancer while still standing next to you (in other words, don't move Takumi to a different square). You and Takumi will attack him together. Then choose Corrin and attack the Lancer to kill him.