
At some point after Wolfhunter launch this achievement has become bugged or possibly has always been. I do know people who have done it the first week of Wolfhunter, but have not been able to replicate it now.
While fighting Mylenne she gains damage mitigation and enrage from each Shock Warden that spawns. Killing the Shock Wardens but not cleansing Mylenne in the aoe shock cleanse the wardens leave behind, as per the achievement states, does not remove the mitigation. Eventually Mylenne will gain 100% damage mitigation and be immune well before she can be killed.
We have tested it slow playing the mechanics and dealing with Shock Warden spawns at the various HP thresholds of Mylenne, and we have done a 3 dps straight burn from the start. In all instances we tested, Mylenne has become immune to damage before we could kill to complete the achievement. In the current state Wolfhunter 4.1.7, we have found the achievement to be unobtainable.
@ZOS_GinaBruno