Make a save! If you have Potion of Clearance skip step 2.
Buy a Potion of Clearance
Novigrad - Merchant NorthEast of Hierarch Square in the building on the West
Skellige - Gedyneith where you do the Practicum in Advanced Alchemy
Use the Potion of Clearance and respend your Points in Alchemy, Focus on
Heightened Tolerance | Aquired Tolerance | Side Effects
Forget about the Decoctions because they bring far to much Toxicity (80).
Chuck your Potions that give 15 Toxicity:
Black Blood | Cat | Killer Whale | Pheromones
With a bit of Luck you already get your Achievement through the passive Effect of the Skill "Side Effects".
If not, we have to advance to the Potions that give 20 Toxicity like:
Swallow | Tawny Owl | Maribor Forest
Black Blood + Cat + Killer Whale + any Pheromones = 60 Tox
Swallow + Tawny Owl + Maribor Forest = 60 Tox
So in the worst case where "Side Effects" does not proc once, you need a Tolerance of 120 Toxicity.
If you have the Pops Mold Antidote - this will also work and only adds 15 Tox (you get this during a Quest)
After all this you can just reload your save with your new Achievement! ;)