Battle Skill is a quest unlocked as soon as the player has completed Defeat Undead.
The objective is to get used to the mechanics of troop formation. It is completed as soon as the player clicks on the "Adjust"-button in the lower left corner of the "Troops"-window and changes the adjustment of the troops. Troops can be placed either Rear, Center or Front, with one slot reserved for the player and one slot remaining empty.
Click on the [Troops] button -> Click on the [Formation Adjust] button located at the lower right -> Adjust your formation once. Try using different formations for different enemies.