Blue Mountains. Since there is a huge variety to the Stonemen they can be very difficlut to defeat. Most of them are capable of making a Falling Rock appear in their hands. Those can be very difficult to avoid since they can be difficult to see comming. When they are punched, they explode into six pieces that slowly spread out across the screen and even outside of it. Some others can shoot blue flames onto you that quickly spread out across the screen.
While some stonemen prefer to stand still (as that one on the picture) others can jump away from you to start with another attack.
They can be found in Mystical Pique.