Perhaps it isn’t a big secret that you get 450 gems for reaching 3 stars on the “Sweet Victory” achievement. What could easily be more of a secret is that this is really easy to do. Starting as low as a level 6 Town Hall (maybe even lower), you can search for villages which have a town hall outside of their walls. Taking out this one building guarantees you a star and some trophies. When you are just starting you might think “why would anyone do that?” See Leaving you Town Hall out in the cold.
Though you may not yet understand why, plenty of opponents do this – and you need only search diligently until you find such a target, and then take your cups and smile.