Are you feeling short in experience using any Elemental Ghosts? Worry not, Brave Frontier supports evolution for your Ghosts and turn it into King. It follows the normal evolution mechanics but requires the following materials;
1. All Ghost Elements, depending on what Elemental King you want. If you want a Fire King for example, you will need all other Elements instead including your existing Fire Ghost.
2. 1,000 Zel cost for evolution
If you a want to know more and how to evolve your units please read the article here.