This achievement is story related and is unmissable.
You unlock this during the main quest Evil's Soft First Touches after killing the Toad Prince in the Oxenfurt Sewers.
The Toad Prince is vulnerable to Cursed Oil, Golden Oriole, Northern Wind and Yrden.
Apply Cursed Oil to your sword and drink a dose of Golden Oriole at the beginning of the fight. Dodge his attacks until he is close to you and strike him with your sword. If you would like, you can place the Yrden Sign before you begin to hit him and throw the Northern Wind bomb at him too. Don't forget to protect yourself with the Quen Sign as the Toad Prince does a hefty amount of damage. Repeat this until he is defeated.