This one is rather simple. Either you get a high enough explosive level to unlock the singularity grenade and lob it at 3+ enemies or you go for the Oblivion cannon.
*** Spoiler - click to reveal ***The Oblivion cannon can be found at the top of the science Centre which is a group of 4 skyscrapers just slightly south west of the New Pantheon building. Once you have made your way to the top and disabled the shields, you should be able to just pick it up. The first group of enemies that spawn in should be at least 3 large, just fire the cannon at em and the cheevo is yours! (More guys spawn in if you miss the chance on the first group for some reason)