Nightmare Avenger Trophy
Complete the Story Mode on Nightmare difficulty
After beating the game on Easy go to the main menu and start a new game, Nightmare difficulty will now be unlocked. Play through the entire game and defeat the final boss for this to unlock. To make this trophy easier, avoid any unnecessary fights and skip through the cutscenes. It is also a good idea to have a few different Skylanders in case your main Skylander gets KO'ed, I highly recommend having at least 4 Skylanders but it is not required. What you want to do this playthrough is try not to get hit and you are golden.