This achievement involves killing Kologarn without destroying any of his arms. Today we found out that using Guardian Spirit on the person grabbed will completely disable the mechanism that kills the grabbed target after a while. The GS'd person keeps taking damage, but will stay in the hand forever (obviously no new people will be grabbed either). This completely trivializes the achievement because you can ignore the arm and only have to heal the grabbed person.
I've seen it happen in the 10man version but I assume its the same for 25man.