This is an easy one that you can't miss.
Early in the game, the first area you'll visit after you leave the first city is
M.o. Metal. Towards the south of the first part of the map (Coordinates 5,9-3,11) is a 3x3 area depicted with a yellow checkboard pattern on the map. This is A Hiding Area.
Enter the hiding area and then press the
button to access the menu. Choose
Hide, confirm you want to spend the required Morale, and press
until a group of enemies attacks.
Accept and win the battle. Whether or not you beat the leader, you'll still get the Achievement. If you do defeat the leader, you'll also receive a treasure box.