So here it is. If you travel to "Outlying Settlement" (preferably after unlocking all destinations in the area first) in the northeast corner you will see a destination called "Eastern Path" now what you want to do is travel to that point. if you entered outlying settlement at that point it will still work. all along that northern road starting at "eastern path" and ending in the "Trail to Fort Hope" area there are spaniards patrolling the path. There are only around 10 but if you travel somewhat slowly along the road and pick them off with your bow or crossbow, or even your knife if youre tactical about it you should be farming steath kills left and right.

Once you have eliminated all the sentries along the road all you have to do is open your map and fast travel to "Eastern Path" but this time leave the "Outlying Settlement" area via the eastern path. Once its done loading you should be facing a gate in the next area. Just walk straight through that gate and it will reload you at "Eastern Path" again and you can repeat the process. All enemies will be reloaded just waiting for you to pick them off.

It is a bit trickier if you are going for the "Backstabber" achievement but it is very possible. If you mess up the groups are far enough away not to alert each other so just eliminate them asap and then move on to the next part of the road. and reload to "Eastern Path" when you are done.

As I mentioned earlier this is also a decent area for the "Antagonist" achievement as well considering there are not a lot of good areas to farm enemies in this game. completeing the story only netted me around 600 personally but with a combination of guides and this tactic here, getting the final kill based trophies was a breeze.