For this achievement you'll need upgrade your Sour Tower so that it has 6 pieces, you do this by either smashing a sour piñata once it's visited your garden, if you choose to do it this way you'll have to pay for your sour tower upgrade, alternatively you can try and turn the sour piñata into a normal piñata, if you do this the tower upgrade will be free.
There are a total of 8 sour piñatas in the game, here's a list of them and what they need to turn into a normal piñata;
Needs to eat an apple seed
Needs to eat a Jack O' Lantern
Needs to eat a red flutterscotch, you'll also need 2 mushrooms in your garden
Needs to eat a Pigxie
Needs to eat a Cluckles, you'll also need to have 5 master romancer awards
Needs to consume a bottle of medicine, you'll also need a bird bath in your garden
Needs to eat 1 Sweetooth and 2 Swananas, you'll also need at least 160 Pinometers (16% of your garden) as water
Needs to lose in a fight against either a Sryupent a Twiggersnap or a Fourheads
For more information on the sour piñatas please see here; http://pinataisland.info/viva/Sour
, you'll find all the information you should need there.
Below you'll find videos which show you examples of these methods and of the sour piñatas changing.* Credit to AnInsaneGuy for the videos