To get this trophy you need to clear all monster nets (can be found at "?" locations and are marked with a monster head when discovered) in Velen/Novigrad or Skellige. Skellige is the best zone to do this in having that there are more nests in Velen/Novigrad than in Skellige.
There are a total of 5 nests in Skellige:
There are 4 nests on the main island (Ard Skellige) and one on the northern island of An Skellige. Having that the quest to get to Skellige is level 16 its recommended that you are around that level.
Refer to this map:
http://witcher3map.com/s/#3/24.13/-4.31Clear all markers on the left and just enable the "Monster Nest" one. You could enable the "Sign post" one as well to see where the closest travel point is.
If you are dead set on doing this trophy in Velen/Novigrad refer to this map insted:
http://witcher3map.com/v/#3/60.00/-5.00