Credit foes to ShadowWolf810
Steps to Manifest Destiny Achievement
-Get access to the northern part of your map known as the "Great Plains." You will eventually gain access to that region by playing the game!
-Go to the buffalo picture on the map at around 1 am. A huge herd of buffalo should be peacefully grazing there.
-Sneak up quietly and try to take out as many as you can before you spook the herd and they start to stampede.
-Kill as many as you can, you can still come back afterward if you don't get them all and repeat the process.
-Once you've killed all the buffalo, you'll get the achievement.