Swim to the Boss' head first and position yourself over one of the holes on either side. Select one of the globes inside. If done correctly, your Sparkle will dive inside. Once there you can swim around by using the movement keys (not clicking or tapping) and eat only the globes inside the head. This will end the level and earn the achievement .

You will have to continue playing after completing the game. Grind the "Vast Field" level for food. There are no other layers in the level and it features plenty of food.