If you are an alchemy build (
http://www.gosunoob.com/witcher-3/skill-calculator/?alchemy=...) you should already have the required talents. (Heightened Tolerance, Acquired Tolerance and Side Effects) If this is the case, skip to step 3.
PLEASE ENSURE YOU SAVE BEFORE DOING THIS!Step 1:Buy a Potion of Clearance.
Novigrad - Merchant north east of Hierarch Square in the building on the west. Here
Skellige - Gedyneith where you do the Practicum in Advanced Alchemy. (Here is a link to help you do the quest if you have not
http://www.gosunoob.com/witcher-3/practicum-in-advanced-alch...)
Step 2:Use the Potion of Clearance and gain the talents Heightened Tolerance, Acquired Tolerance and Side Effects.
Step 3:Use Black Blood, Cat, Killer Whale and any Pheromones (Drowner, Nekker, Usine). If you for some reason do not get any extra potion effects, use any of the other potions you have that have 20 toxicity. (Swallow, Tawny Owl or Maribor Forest)
After doing these steps your achievement will unlock. If you prefer a video, here ya go:
If you guys have any suggestions let me know. Good luck!