Complete the Greenstone Village scenario without using any Volatile Greenstone Brew.
Part of the Pub Crawl meta-achievement, which awards the Honorary Brewmaster Keg.
In the final stage of Greenstone Village, you will be granted an extra action button which will activate a damaging AoE spell when pressed.
To obtain the achievement, you must complete the scenario without ANY of your party using this ability. So if you are in a random group and want this achievement, be sure to tell the rest of your party.