This is a tutorial stage; if you played Fire Emblem Awakening before, the battle system is pretty much identical. Don't move your avatar; just attack the Nohrian soldier in front of you so that Hinoka can still provide you with support.
After defeating the enemy soldier, read through the next dialogues until your side's turn. Hinoka will move out to scout and Sakura will heal the avatar. A lone enemy lancer will appear in front of you. Like before, you don't need to move; control Takumi and have him snipe the lancer since your avatar's sword is weak against the spear. Finish the lancer off using the avatar to complete the quest.