You have to unlock all 42 challenge events and then play each event after unlocking it. When you play a challenge event in any game mode (except practice) you unlock the next event in the sequence listed here (event #42 unlocks event #1): Solution for All Your Game Are Belong To Us in Viva Piñata: Party Animals
I recommend you play long games by yourself, either local or online. A long game of 7 races with 3 events will have 21 unique challenge events, so you will unlock half of them on your first game. Just let the AI win, that way you do not have to race or take part in the events and if it is an online game you do not even have to press A between races and events. You can check which events you still need to unlock by starting a local game and selecting the practice mode (which you can toggle between races and events). The events you have yet to unlock have ????????? as the name. You have a 1 in 42 chance of a particular event coming up. Your chances increase when you play a long game as the odds become 1 in 22. It took me 20 hours to unlock the last event I needed. Once you have unlocked all the events you will need to make sure you have played each event after it had been unlocked, start with the event you unlocked last.
When you start working on this achievement, print a list of the events and put a tick in front of all the events you have unlocked (as shown in practice mode) then each time you unlock another event put a tick in front of it (remember it is not the event you just played that unlocks, but the next one on the list). Then each time an event comes up that has a tick in front of it, cross it out (remembering to tick the next event on the list if it isn't already ticked or crossed out.
I had a problem getting this achievement. All the events were showing as unlocked (none of them were ?????????) and I had played through all of them in practice mode, twice and still I didn't get the achievement. Only when I loaded the game on a different console did one of the events, number #6, show up as ????????? As soon as I had played event #5 (20 hours later) I got the message that a new challenge event has been unlocked. I played event #6 in practice mode and finally got the achievement.
You may have noticed Viva Piñata: Party Animals does not have any game saves. Clearing the system cache or deleting your profile also does not reset your progress. The events you have unlocked are stored in your profile as when I recovered my profile to another Xbox it knew which events I had unlocked.