On map Deathspin. Go to the bottom right dead-end area, and keep the town and knight tower destroyed while collecting powerups. This is your "safe" area for the map ("safe" being relative to the enemies that are spawning). Your main goal is to destroy royal carriages and allow knights to rescue them. You can destroy them across the level boundaries, and ransom princesses nearby if you run low on health. Once you get 12, the achievement will pop up.