What happens to M.atk when big banghere?

I hear that when big bang happens, M.atk is for w.atk users as well. if this true? if so like how will this work?

Yes, it's true. It will work exactly like Weapon Attack, so um I'm not sure what else needs to be answered. Mages will have a damage range like every other class, and % damage on their skills.

Edit: Oh, I can't read. No, w. att classes will not use m. att.

w.att users wont need it but mages will use it as other classes use w.att.

Magic ATT is useless to Weapon ATT classes. You most likely misinterpreted what you heard. What's really happening is that Magic ATT will be to mages like Weapon ATT will be to everyone else; it won't be equal to INT anymore.