Strategy:

• This boss is an interest one. First, it is resistance to physical attacks.
• Whenever you attack with an elemental attack, it'll store energies of that element then gaining resistance for it by half meaning you only deal 50% damage next time for the same element.
• After 3 hitting 3 times of the same element, it'll release the elemental attacks it stored at you dealing multiple damages. Then its resistance reset to 100%.
• So the most effective way to deal damage to this guy is to take turn dealing different elemental attacks before same element hit 3 times. For example, you can attack with Fire, Lightning, Dark then repeat.
• As you long you keep your HP high, you should be able to soak up the damage.