Let's keep it simple because this is an easy achievement. Make sure you have at least 5 stars at the ready. If you don't know how to get stars, head to the quest board in your base and complete either Plant or Zombie quests to gain stars. Do not select Multiplayer as it only gives you coins. Once you have 5 stars, head to the sewers and go right, then left and down the ramp, then take a right and 2 lefts and you will be staring at the fish. Hold
to spend your 5 stars for 750 XP, and then the achievement should pop up!