Resistance 3

"Environmental Objects" simply means blast roots. The easiest way to rack up kills for this trophy is to shoot blast roots when an enemy is near them. This will always kill the enemy and count towards this trophy. This can be done throughout the whole game, as blast roots will appear in almost every level. The best place to farm this trophy is at the checkpoint: Haven, OK: Flooded Town. There will be a large amount of Grims on the rooftops, accompanied by a fair amount of blast roots. Shoot the roots, not the Grims, and the explosions caused will chalk up several environment kills.

Quit to the menu, reload the chapter, and repeat this process. Kills carry over across multiple playthroughs, and your progress can be tracked via the Extras menu, under the Rewards section.