Resistance 3

As you gain the Wildfire weapon (basically the rocketlauncher of your arsenal), you will straight away be given the ability to use its Secondary fire which shoots out one rocket, then exploding into multiple, basically a cluster bomb.

Keep the ammo on your wildfire handy if you can once taking out the thing it tells you to once you obtain it (avoiding spoilers as much), you will soon later on after that part, come across some resistance fighters, who in order for you to get where you're going, you need their assistance, in which they need yours.

They'll soon send you back out again to go killing, which when you do, you get introduced to a knew bunch of grims via mini cutscene, kill the first bunch it introduces you to, then continue your way around the house which is the necessary path you have to take, as you come out into a bigger opening, you'll see a group of grims together, who will soon run different directions and escape/hide.

Walk up into the middle of this area and wait around for a few moments, avoiding death is possible. Not too long after, you'll be bombarded by 7-8+ enemies at once, just simple use your secondary fire (R2) on the entire group to kill them all and pop the trophy.

If this doesn't work first go, just die and retry. Or continue onwards through this level, as there are a lot of bits where you'll be swarmed by grims.