There are two ways to complete this; one is to invite a friend into a free roam and start throwing tomahawks at each other from various distances. The other involves throwing tomahawks down onto enemies from a great height. This achievement will require a lot of trial and error so be prepared to spend a few minutes with various trial and error attempts.
If you don't want to do this with someone else, head to Pike's Basin, start the hideout and allow yourself to die. You will now start at the very top of the canyon, instead of heading down, head to the far right of the canyon where you will find the furthest most objective. Stand up on the ridge and pick a target. You're now going to want to aim about an inch above the targets head. Throw a Tomahawk, then move your crosshair up if you miss. Keep doing this until you hit the enemy below.
Attempting this on flat ground can be quite tricky and require a lot of trial and error. With the Pike's Basin method you will already be well above your enemy and have a good 100 yards or so, you just need to hit the enemy.