This achievement requires careful timing and aim so I would recommend that you practice in the practice range first.
What you will want to do for this achievement is knock someone off the map using concussive blast, this can be anywhere on the map as long as they hit the death barrier as a result of the blast.
The most effective maps for this purpose are ones with plenty of open space and focus on areas with drops such as ilios well or Lijang tower garden for plenty of space to fly and central points with a large amounts of area to knock enemies into.