Here is the workaround from that other board:
Quote:Contacted Activision about this, here was their response:
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:
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
All I'm saying is that rather than moving your save game around I bet you can achieve the same results by:
When starting the game it will prompts you to select where your save game is(hard drive, cloud,usb drive) you just hit B and cancel out of that selection.
When you do this and go into play the Adventure level it will look like you have no save games and force you to start a new game to play(it also doesn't actually save progress in this case once you leave).
Follow the instructions about playing to the point of getting to Persephone where you can purchase upgrades and then buy the last upgrade(needs to be an upgrade that doesn't requires the Soul Gem according to above).
I'm guessing this should allow you to unlock the achievement as well but without the nastiness of actually moving your save game. No guaranteeing but I'll probably try this myself over the weekend and report back.