This is the only trophy that won't come naturally.
Your best bet is during the boss battle on Campaign level 6. It's a big black shape with an orange target area. To the left and right of that orange bit are too black bits that stick out. If you shoot those, they'll allow you to hit the orange bit. YOU DO NOT WANT THAT.
Instead just stay at the bottom of the screen, in the middle. And keep shooting. Enemies will materialise every few seconds and you'll kill them for 4-5 kills at a time. Stay there holding down the fire button until you get to 500. The trophy will pop immediately.
I did it all on Easy. Was fine but this trophy did take two or three goes. No worries.