For efficiency of getting this achievement I find this is quite a good method. After completing the 6th or 7th mission to the dlc you'll be introduced to royal demands.
Go to the pigeon coop marked in the mission or just a pigeon coop in general and select Meath : death sentence, continue with this royal demand and complete it, you just have to clear out a camp. Once completed fast travel to Dublin, there is a pigeon coop and also a dock fast travel point next to it. Interact with the pigeon coop and Meath : death sentence should reappear, select it. There should be a fast travel point right next to Meath : death sentence location. All you have to do this time though is walk in the camp and walk out, as the game doesn't spawn any enemies since when you last cleared it out. Rinse and repeat 10 times.