For this achievement, collect 1000 gems and 1000 gold from the secret fountain in stage 2.
The secret fountain can be found on the 3rd big battleship (the one that's horizontal). The ship has some small turrets on both sides. The last 2 turrets (one on the upper and one on the lower side) have the reveal the fountain when they are destroyed.
However, they only appear if the stage 2 mid boss is killed very quickly so as soon as he spawns kill him by using guard barrier.
The fountain will usually give you gems but if you want gold, you need to have reached your highest multiplier (i.e. 500x in Xbox 360 mode) at anytime before reaching the fountain.