Despite the achievement's description, you don't actually have to finish the game to get this. The only requirement is that you kill at least 1200 creatures/kith. If you are playing through the game as a completionist, you should get this naturally. I got it after killing everything you can't negotiate with in Od Nua, everything hostile west of Dyrford, and doing just about every quest possible before act 3 in those areas. There is plenty of room to skip quests, just make sure you do a lot. If you have the White March expansion, this achievement should be even easier.
If you have any ideas on how I can improve this solution, I'd love to hear them! There may be something that isn't here that should be, or an easier way of doing this. I'm always open to suggestions. If you notice any typos or poorly phrased sentences, please let me know.