From Destinypedia, the Destiny wiki
Much like its minor counterparts, these Servitors will hover around a nearby crew of Fallen troops and provide them with immunity shielding thanks to their Ether Shield ability while providing supporting fire for them with their Eye Blasts.
However, they also have another trick up their sleeve; once they lose a significant portion of their health, they will envelop themselves in a personal dome shield that protects them from outside gunfire and slowly repairs them. Still capable of firing their Eye Blasts at their enemies and protecting their allies with the Ether Shield ability, these Servitors and their barrier shield tech pose a significant threat.
To negate the threat of the Servitor and deplete it of its protective shield, a Guardian should utilize Anti-Barrier rounds from the Seasonal Artifact which can penetrate the Servitor's shield and briefly stagger it.
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