To get it you need to first research Eye of the Patriarch for your dragon (it cost 40 research points) and equip it. The best way to get it is to find a cluster of troopers and grenadiers, fire normal attack once on each of them to soften them up and then shot with Eye of the Patriatch.
A little tip, if you have game in other language than English sometimes achievement say Eye of Patriarch but that ability is named different in the game. For example in Polish version it's named Serce Smoka (Heart of the Dragon). This can create some confuson. Eye of Patriatch is basically a big explosion similliar to that of Imp Bunker Buster.