To defeat the thunder demon,you will be outside the museum.It will be attacking you with thunder bolts.To avoid getting hurt,go under the trees.If it follows you keep going around the tree.To attack it ,while its charging for the sky bolt attk, take a rock and throw it at the demon(thats what i did). It will be stunned for a few seconds.During that time attack it with everything you have got .Repeat the process till it is over.