Start Rookie Cove. You can find fish that is not moving in 4 places of this area, on the bottom of the sea. Fish spawns in the same order all the time. Shoot the head, so you will get Headshot or Stoned bonus and you will not have to fight with the fish. Go to the next spot and repeat. First one is below the smallest coral, 2nd and 3rd below the far away coral and 4th below the biggest coral on the other side than level start. You can easily get The Stoner, Head Hunter, The Predator and Maximonium during the process.