This achievement is for completing the main story quest Poisoned Ground
, which is available once you complete the following mission:Flint & Tinder
You will be required to sabotage the fort. Simply place the poison in barrels using
. It should cause no problems, but you can always play on easy if you are having trouble.
As the achievement description states, this happens in Seregost.
This has been noted as failing to unlock for several people. An unconfirmed solution has been posted in the comments. If one can confirm it works, please comment.