Resistance 3

The Atomizer is first obtained in the Trainyard in St. Louis, MO. It's secondary fire produces a makeshift black hole that shocks enemies, and sucks them towards the center. Immediately upon receiving this weapon, you will be inside a warehouse loaded with Grims. The best way to get this trophy is to get as many Grims on you as you can, then press to shoot out the black hole. All of the enemies in the immediate area will be killed almost instantly. You simply need to get 30 kills using this.

The trophy can either be farmed here, done throughout the game. The progress towards this trophy carries over throughout multiple playthroughs and chapter select, so you don't need to worry about missing it. You can check how you are doing towards this trophy through the Extras menu, in the Rewards section.