In order to be able to do this, you first have to beat the story mode. When you have beaten story mode, go to the main menu and select extras. Load the save file with your completed game. Click on "dragon challenges" and beat all five of them. When you are successful the game will tell you that you have unlocked Dark Spyro. Go back to the main menu, load your game again and save it. Use the the element change button until you reach Dark Spyro (it's the last one in the elements sequence).
When you go up against the huge armored scorpions and you cant beat them keep dying and fighting people till your fury bar get full and get really close to it and unleash your fury attack and it should kill it.