The likelihood of obtaining this achievement naturally is slim as this takes patience, precision and a little luck.
Firstly you need to the Crossbow. I would recommend maxing out the upgrades on this as this not only increases damage but also increases the arrows flight speed. For one shot kills at long distances these are pretty essential. The 4X zoom is also essential and the lasersight is very helpful.
Next you need a target. Ideally a sniper or guard based in a stronghold is ideal, they tend to be pretty stationary giving you time to line up your shot.
Getting to a high vantage point is very helpful. If you are on the ground and your target is on above you, so you are shooting up, it will take you forever, I tried almost ten minutes. At the very least you need to be on the same level. Ideally though if you are higher up then the flight speed and distance of the arrow is increased. You need good visibility of the target, if all you can see is there head you will be making this tougher than it needs to be. If however there full body is showing then this increases the target.
When you have a target you need to guage the distance, there a various tips, You can set a custom waypoint on the map at the location of your target this will give you a rough idea. Alternativly you can actually get a good sense just by using your 4x scope. If when zoomed in the target still seems far away then you are likely more than 50m away.
Another method is to wait for a flashpoint to appear on the map, when this happens the enemeies are all usually marked on the map, with targets indicating their distance above there head. However you have to be pretty quick and the enemies are usually moving around.
Finally the kill shot. You will be used to simply aiming the crossbow and as long as you are on target getting a one shot kill. Not at 50m, if you aim for the target even when fully upgraded the arrow will fall far short. My method was get the target in your sights, then raise the scope so you are about 1-2ft above his head then shoot. If you miss look for the stream and impact and adjust accordingly.
This achievement relies on fining the right location, the right target and mastering the long range technique.
I obtained this achievement on the Lombard map which I prefer as enemies dont tend to roam the streets much leaving you free to take shot after shot should you miss.