I ended up trying to get this achievement over 10 times, but the best i could do was $735. Did a quick search and saw there is a super easy way of getting this.

I'm probably one of the last people to play this game, but this achievement only has a 17% completion

When you get to round 4, you will see a ship with some tentacles and lights. Once the round starts, from right to left, shoot the white lights. It will activate a little shooting target fish that will pop up 3 times. Each time you hit it, it will credit you $200, for an easy $600

(Sorry for the blurry screenshots. For some reason when i take screenshots, they turn out just black, so i had to take screenshots from a youtube video)