Mocking whisper is the copy version of your operator / drifter that comes out of the mimic'd warframe that laughs and shoots grimoire blasts at you after finding a grimoire in mission right? If so just had that...and after killing the "Shell" the operator /drifter form is undamagable by everything. Not sure what to do to damage it either. It just is incapable of bein damaged. Warframe abilities, weapons, operator abilities weapons. Just seems impossible to kill.