Bugged Achievement.
This need to be done in ONE playthrough
DO NOT CLOSE THE GAME.
For some reason achievement progress restart if you close the game.
Bag 250 Outlaws alive. You can quick save, bag outlaws and reload the progress continues even after reloading the game.
For example :
Quicksave....Bag 2 Outlaws....Reload....bag the same 2 Outlaws now you have 4 reload then continue like that until 250.