how to unlock the secret characters in ww2 zombies?
['Survived 25 Waves and Prologues. [06:45]', 'Defeat the Panzer Mortar for the First. [11:23]', 'Unlock the Weapon Upgrade Station for the First Time. [11:50]', 'You Need To Defeat the Panzer Mortar by Wave 12. [14:42]', 'Defeat the Panzer Mortar with no Blitz. [15:20]']