In Chapter 3, A Burden of Blood, you will learn the alchemic recipe Exstinguis. This happens in part 3 of the chapter, Rodric's tool. With this item you can extinguish flames from a distance. For this achievement you need to extinguish 40 flames. This can be done through normal gameplay, but if you are in a hurry to unlock this you can grind out the achievement by selecting restart section from the pause menu anywhere there are fires. Just put the fires out, restart section, rinse and repeat until it unlocks.
In the video guide I created for this on my YouTube channel I show how this can be done right after learning the recipe. Following this guide you should be able to knock this out in about 5 minutes: 2 fires per run at about 15 seconds per run, 8 fires per minute. Messed the math up in the video and came up with 10 minutes (what I get for not scripting it out before recording).
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