There's an easy spot to get this in Chapter 2, level 1. Go through the level until you reach a checkpoint. Keep going for a bit and you'll see some stairs leading up, you can get another trophy here:
Solution for 2 For The Price Of 1 In Killzone HDKeep going after this and you'll reach a room with 3 friendlies near a console. As you move past them they'll say "Enemies coming from the right!" Make sure you have the M82-G equipped (it's the starting weapon).
As soon as the friendlies yell about the enemies on the right, keep going past them and around to the door where the enemies are coming from. There should be 7 of them. Once they pile through the door, aim at the middle of the group and you should get this trophy. If not, restart from the checkpoint, get back to the room and try again.