I was surprised to see how many people got this achievement when the game is quite demanding. But after finishing it, I realized there is a much easier way to unlocking this. Just finish Lost City, all the way to the last level. There's usually 30 levels per world. When you do, you will unlock the option for using the detonator, which allows you to blow up grenades at will. This makes things a lot easier and you can easily get 3 stars on the remaining levels.
You just have to complete the world regardless of the number of medals you get on each level, in order to get the detonator for that particular world.