You can purchase upgrades for your Skylanders from Persephone. Save your money to purchase only upgrades, and don't buy anything from the shops until your Skylander is maxed out. You should be able to buy all upgrades by level 12. See "Spend That Loot!" for a way to get potentially unlimited money.
Alternatively, if you have a Skylander with all upgrades that you used in Spyro's Adventure, you can place it on your portal, and take ownership of it in Giants. The achievement will unlock immediately.