Pest Control

 is an achievement in and requires one to destroy all the monster nests in the Velen/Novigrad region or Skellige.

Your best bet is to destroy the nests in Skellige, as the islands only have 5 nests, while the Velen/Novigrad region has 28. However, if you're vigilant about uncovering every hidden location and completing each one, you should have this unlocked by the end of the game.

Tips

 * Make sure you always have bombs available to take them out.
 * Sometimes, if you wait too long, more monsters appear, so be sure to quickly plant the bomb if you don't want to face more opponents.
 * This goes hand in hand with the other achievement Fire in the Hole.