You need to kill 3000 small or medium sized Akrid. These count across all your playthroughs on any difficulty and it doesn't matter if you're in co-op or solo. You can check how many of these kills you have by going into My Page > Statistics > Campaign Statistics. They are listed under "Enemies Defeated (S-size Akrid)" and "Enemies Defeated (M-size Akrid)". Unfortunately, there is no "total" for these stats, you will have to count each section across all difficulties to get a count. If you can find a group of small ones flying together make sure to shoot them, as that seems to be a good way to rack up some S-size kills.