This can be unlocked at pretty much any point throughout the campaign. Two early points that stand out however are Act 1-3: Giant and Act 2-1: River. During the former you will encounter dropships that provide an opportunity earn this achievement as you can destroy the disembarking troops before they leave the ship and disperse. During the latter you will immediately encounter a number of Gorgie-USN and Chicane's blocking the path that are grouped closely enough together.