Town Hall level 6 is necessary to unlock the Healer, a flying troop, at Barracks level 8. She is the only trooper unable of assaulting, yet she is still able to heal any soldiers on the ground.
Strategy
Offensive:
Make sure you destroy or otherwise occupy any air defenses before sending in your healer(s) because a healer will not stay long if she is within range of one.
Sending an indirect attacker, such as a Bowler, Electro Dragon, Super Wizard, or Super Archer, to shoot the Air Defense before it can be used is one way to eliminaate them.
Defensive:
If a healer is within the range of an air defense or single-target defense, she won’t last very long, so make sure to fortify and shield it as much as you can.
Additionally, ensure that it is slightly off to the side and sheltered by walls, away from any direct threats to troops. By doing this, they may be certain not to target the air defense.