You will need to buy 3 costumes for the following:
It will cost you 1500 for each character costume (500 per costume). In total you will need 9000 to purchase all available costume.
Two methods in getting this achievement:
1. If you are short on coins, replay level 2. Midway through the level you will need to clear a path through some kelp plants using the Sandy power up.
There will be an infinite spawn of robots (3 robots at a time). Stand over the Sandy power up (don't move from there) Do not clear out the kelp plants. Instead remain standing over Sandy power up and let the 3 robots come to you. And you will constantly be getting coins.
This method isn't perfect. From time to time, in between picking up the power up, there is a chance for the robot enemies to bump into you causing you to move from standing on top of the power up. So just keep an eye on your character.
2. This will only work if you have exactly 9000 coins at one time.
Go to the closet and purchase all available costumes for the characters. Once the achievement unlocks > Dashboard the game and when you load up your game again you will have all your coins.