Healers are Lumes in Sanctum 2 that acts as its name suggests: A supportive Lume to restore other Lumes' health. They mainly appear in the Road To Elysion DLC pack but can be found in other DLC packs as well. Healers should be targeted first to lessen the amount of damage needed to stop a wave of Lumes.
Healers have no method to attack players and have low health. They have fast flying movement and can fly over bases, but tend to follow the path that ground-based Lumes take. The player can simply eliminate them by shooting them anywhere on their body since they have no armor. Their weakspots can easily be found behind them. Most Towers can do the job just fine but the Anti-Air tower is suggested if you are playing The Last Stand DLC. A Healer will typically fly behind another Lume to heal it, but if that Lume is killed, the Healer will quickly retreat until it can find another one to heal.