If you did not get this during the story campaign. Just load up Mission 14 and build a total of 50 towers or more.
You can also unlock Annihilator, Gun Crazy, and Siege Breaker using the same Mission 14 map.
See the appropriate link below:
Defense Grid 2
Full Defense (BRONZE)
Build 50 towers of any type in one mission. #PlayStationTrophy