How do I get the mermaid achievement in Kingdom Rush: Frontiers?

The mermaid can be found in the fifth world "Buccaneer's Den". She doesn't pop up frequently, so you'll want to earn this is achievement after you've defeated all three levels (Campaign, Heroic Challenge, Iron Challenge). Keep your eye on the left hand side of the screen, she pops up under the boat near its anchor. Quickly click her to earn the achievement.