Monster nests are one of the little side things (not quests) you can do in the game to occupy your time and gain experience. They're the ones you have to drop bombs in to destroy. There's 23 in Velen/Novigrad and 5 in Skellige. So if you can wait until the main quest line takes you to Skellige, it's way easier to get this trophy there. As for where to find these buggers, here's a link to a really helpful interactive map I've found. Happy hunting!
http://witcher3map.com/