After defeating Kaos in the final level of the game, the Crane Deck becomes filled with gold (between 6000-8000). If you pick up the gold, then press Home and return to the Wii U menu without saving, the Skylander will keep the gold, but the game will return the gold to the Crane Deck since you did not save. You can repeat this as many times as desired to quickly unlock all the moves for all the Skylanders, and purchase any hats/charms that are expensive. Note: Remember to pick up the Winged Sapphire from the Crane Deck after Level 15. Picking this up saves the game. If you touch it after picking up the gold, the gold will not re-appear.
Nightmare Difficulty
This mode becomes available when you complete the game on any level of difficulty.