I made a monster cake for my son’s 5th birthday last year. I made it with two store-bought cake mixes, and prepared the cake mixes as directed. I then poured the batter into an extra large cake pan (15 inch round?), and some into a regular 8 inch round pan.
Once baked, I stacked the cakes and frosted them with green buttercream frosting. All the decoration was done with pre-made fondant. The cake was a hit and there was plenty for all!