Avoid its attacks, especially since he uses fire based attacks. If you get burnt, used the Gale Storm Brush. When he stands up and burst into flames, attack him using your Devout Beads. He will reveal himself as a flaming skeleton. Use the Gale Storm Brush to put the fire out and make him just a normal skeleton. When in this state, attack him using your Devout Beads. Continue doing this to defeat Crimson Helm.