Metal Robo Na Hanna has a very powerful magic. The magic he has prevents him from taking damage, and to make it worse damage normally done to him is done to you!
The only way to defeat him is to use the item "Mr. Virus" which steals his invincibility. The item "strategy book vol. 8" says it steals all powers, but it doesn't. When you get "Mr. Virus" and are fighting Metal Robo Na Hanna use a projectile attack or a projectile secret buster.