You will unlock this achievement for completing the last mission on the main story line: The Son Of Medici.
When Di Ravello's Bavarium shields go down, shoot the RPG 'Fire Leach' which is unlocked by liberating the Cava Geminos Nord base in Rocca Blau province.
Once Di Ravello's Bavarium shields go down, tether his helicopter to the ground and pull. Then his chopper should crash in to the ground.
If you haven't unlocked any gear mods for your tethers, I recommend using the solutuion above.