Dauntless

First, you'll need to obtain the Parasitic perk to make this achievement work. You should get one fairly easily inside cores that you open in Ramsgate (which are rewarded on hunts, or from the daily fountain). You could also craft some armor from the Agarus enemy, which has the perk on it as a passive buff. Once you have the perk either installed via a cell, or from the armor bonus, you'll need to kill both the Sprestruck Charrogg (which can be found in Terra escalations and on three separate islands) and the Sporestruck Embermane (which can be found in Terra escalations and on two separate islands) to unlock the achievement. These don't have to be done in the same hunt or even the same gaming session.