"This majestic creature lives to protect and aid her fellow troops. Any army is improved with her healing support, but make sure to protect her from air defenses!"
Level 1 & 2
Level 3 & 4
The Healer is a flying unit. She is the only troop with no attacking/offensive capability, but can heal any ground Troops, except when she is defending as a clan castle troop (in which case she heals damaged Buildings, Clan Castlereinforcement troops, and heroes).
When the Healer is deployed, a pulsating circular aura appears around her. If all units have full health, she will start healing the closest troops (although it will have no effect on the healed troops). Should a ground troop be damaged by a Defense, she will fly over and start healing.
A Healer does not last long if she is in the range of an Air Defense, so make sure you destroy or otherwise occupy the Air Defense before sending in your Healer(s). One strategy is to send a few Wall Breakers in to break the wall that is protecting the Air Defense. Then send in a group of Giants (usually 12 or more) to destroy it. Finally send in a Healer along with other troops after the Giants. As long as the Healer does not get distracted by other Troops taking damage, or is not killed by a corner Archer Tower or Wizard Tower, the Giants will live for a significant amount of time (due to the constant healing) and will distract the defenses while the other troops destroy everything. This is known as a Giant-Healer raid and is extremely popular when attacking a Town Hall 6, since a Town Hall 6 only gets one Air Defense.
If at any point in the battle all of your non-Healer Troops are killed and you have no more non-Healer troops or Lightning Spells to deploy, the battle will end, even if you have Healers deployed that are still alive or non-Lightning Spells that are still in effect.
Healers can allow Giants and other defensive troops to withstand massive amounts of damage, even doubling that of their normal health.