Little Help to young Hackerman

Good evening dear members! 
😊


Im trying to crack a wpa2 and i already have read about masks .

Lets suppose that wpa2 password contains 15 digits (i totally dont know what type of characters wpa2 composed of)

What mask do you reccomend me to use since all available dictionaries i used has failed?


Is there any common unfailable method already used? 

Thanks a lot for your time in advice!