Easy achievement to do, similar to the same type of achievement from the Witcher 2.
To deflect arrows back you must have at least two points into your "Arrow deflection" skill (center one) in the combat tree, if you put three points in the deflected arrows will do double damage. And in case you forgot how to deflect arrows it's the same as to parry, the LT button, but of course you have to time it when the arrow fires. *ALSO don't forget you will need to put the arrow deflection skill in a active skill slot in order to actually deflect them* (Credit to William723 for reminding me about that.)
Now the easy part, just find a bandit camp you haven't cleared yet (from this new dlc). There is usually at least 1 guy with a crossbow at every camp. Kill all the other guys around him. Now just parry and deflect his arrows back at him until he dies. Do this to three guys and the achievement will pop.
If your struggling to find bandits with Crossbows, I found a camp to the South of the "Upper Mill" marker. To the South of a marker called "Zuetzer Castle". The bandit camp there had three crossbowman. (For me at least).
(Reloading your save to kill the same person will NOT work.)