This achievement is gained once you have defeated the Toad "Prince" boss in the sewers of Oxenfurt. This is unmissable.
The Toad Prince is a fairly difficult boss; it has fast attacks, with plenty of range and close combat capabilities. Preparation is vital: bring lots of healing items and thunderbolt. Also according to eurogamer.net it is vulnerable to Northern Wind, Golden Oriole, and Cursed Oil. Also, the Dragon's Dream and Grapeshot combo work effectively.
This can be done on any difficulty, all that changes is the length of the fight (because of the relative strength of the boss), so if you plan on doing this on Death March then make sure you are fully prepared for a long repetitive fight.
Follow these steps:
1. Place Yrden, preferably not too far from the boss (but not in range of its leap).
2. Dodge attacks until the Toad Prince makes its way over to you and steps in the trap, you can throw bombs if you wish.
3. Move towards the boss, applying thunderbolt as you approach.
4. Attack three times (from what I found, its stun is lifted after three hits, heavy or light. So you may as well do heavy, as it would do more damage overall).
5. Dodge away, healing if necessary.
It will be unsurprising if this takes a few tries; I personally found this more difficult than the final boss.
Thanks to Kahless Clone for the Dragon's Dream & Grapeshot combo.