From what I can tell, the Badge of the Willbender from the Purifier collection seems to be unobtainable. The achievement says 'Hint: As a guardian, defeat one of the Purist lieutenants in New Kaineng'. There is no Purist lieutenant mob in the game. Me and a bunch of different players have attempted to hunt down all other types of Purist mobs, without finding the badge. Purists in the story, in open world, in events, in strike missions, in the new guild hall, none of them drops the badge. We have even attempted to kill lieutenants of the other factions in the expansion.
I have encountered a lot of players who are stuck on this part of the achievement, and none that have completed it.