With a level 2 Barracks, the Archer is an unlocked single-target ranged unit. She is the second troop unlocked in the game overall and the first ranged troop unlocked in the Barracks.
Strategy
Offensive:
Despite being ranged and hence able to attack across walls, archers are extremely vulnerable to fire. Due to their low health, they are easily defeated by almost any defense, but mortars, wizard towers, and even bombs pose a particular threat because of their splash damage, which may swiftly decimate an archer group.
Defensive:
Because she can target both ground and air units, she is a great troop to have in the Clan Castle.
She is susceptible to Lightning and Poison spells, much like any troops with low health, therefore if your opponent can get her to leave your Clan Castle, they can use just one of these spells to destroy all of your archers. Locating your Clan Castle deep within your stronghold will make it more difficult to entice them out.