I know that the combat AI is not going HAL on me, but I swear, the more of them I fought at once, the faster they seemed to learn how to assist each other. Pull small groups of them. Do not pull rooms.
If you are fighting a group of Guardians, kill the Gallery Repairman first. As he is the healer, this is imperative. If there is more than one Gallery Repairman in the group you are fighting, give up and flee. They will overlap heals and quickly become unkillable, especially as the Gallery Overseer stacks his debuffs on you weakening your attacks.