INVERSUS Deluxe

I was able to get this pretty easily in the first arcade level. There comes a point where a bunch of red squares will all spawn at once. I just tried to sort of corral them in the center while I went around the outside edges. You can actually get a rough count of them while doing this too.

If you count them up and there are at least 20 then just make sure they are in a nice little cluster and shoot them.