How do you nammu skill in Rumble Fighter

Once you reach lvl 20 you must finish survival at least three times. Then a message will appear claiming that you have unlocked the nammu skill. Go to practice, press V then tap X(punch) repeatedly until the combo ends. You cannot use this skill in Exo form though.


*EDIT*
This ability has been updated, so you only have to reach level 15 and finish Survival three times to get Nanmu Skill.