Did u search on here for cauldron cakes. There are many on here to look at and get ideas from. Many which have amazing fires and skill to them. Good luck!
Use isomalt colored yellow, red and orange. Also someone told me that they used dry ice for a grill cake one time. The dry ice would give you a great effect.
Here are the directions for that cake.
Good flames can be made with red, yellow, orange buttercream.