From Destinypedia, the Destiny wiki
They usually behave like a normal Colossus, but have an additional ability: they can walk around while firing their heavy weapon, making them a more versatile target to deal with. Once they lose a major portion of their health, they envelop themselves in a dome-shield, regenerating their lost health while continuing to rain down fire upon their enemies. To prevent them from gaining an advantage from this ability, a Guardian should utilize Anti-Barrier mods from the Seasonal Artifact, which is the only way to break the shield, leaving the Colossus open to damage.
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