You must do this by playing through the Orville levels. The Shark King DOES NOT count toward this goal.
Easiest way to do it is get play a level that has mines in it. Grow to the biggest you can and then just eat sharks. When you are about to pass the level kill yourself. Again try to just eat sharks until you are about to finish it and kill yourself again. Just repeat this pattern. You can have a game over and use your continues just make sure you finish the level before the last life of your last continue. I got up to 100 sharks once using this strategy