To get Great Demon card, after you beat the level Blessoon, it will ask you to pick 1,2 or 3. If your luck is good then you will pick a Fire Golem(if your luck isn't good and you don't pick a Fire Golem you can always get a Sand Golem and use it in battle to get experience and transform the Sand Golem into a Fire Golem). Use your Fire Golem in battle so it gains experience, so you can transform it into an Efreet. Use your Efreet in battle to gain experience, to transform it into a Great Demon. Have fun with your Great Demon.