I have read on forums and experienced some issues myself in receiving the Skylanders’ achievement entitled: “Gotta Buy Them All”. There are several youtube clips showing some trickery for swapping your save files which I have not yet tried. I contacted Activision explaining the situation:
I have maxed out the abilities on several Skylanders that have only been owned by me and have not received the “Gotta Buy ’em All” achievement.
To which Activision responded:
I am sorry to hear you are having this problem.
If you have leveled up multiple Skylanders all of the way and still have not received the achievement, then this may be an issue with your save.
Please note the following “fix” is not officially supported by Activision. This has been known to solve this issue for some people and may be able to resolve this issue for you.
1. You need to have 1 Skylander that is 1 upgrade from having everything unlocked. Please note that the upgrade cannot be the one that requires the Soul Gem.
2. You need to have a USB/external memory unit for your XBox 360
To resolve this issue;
(in the XBox 360 Home Screen)
Select Settings > System > Memory > Storage > Games and Apps > Skylanders: Spyro’s Adventure. You will need to take your save file, and move it to your external memory unit. Once this is done, there should be no saved data in the Skylanders: Spyro’s Adventure folder.
Once this is done, launch the game and it will be as though you have started from scratch. You will need to play to the point where you save Persephone, and can once again purchase Upgrades. Once you purchase the final upgrade (that does not include the required Soul Gem), you should unlock the “Gotta Buy Em All” achievement.
Once you have the achievement, you can once again move your Skylanders: Spyro’s Adventure save data from your memory unit, back to your hard drive. This will allow you to continue from your original save point.
If you have any further questions, comments, or concerns, please reply to this incident or contact us by phone so that we can continue to assist you.
I am going to try and level up one more Skylander, ensuring that I buy his unlockable power before the other upgrades and if that doesn’t work, I will go the route suggested by Activision.
Good luck, Skylanders!