Winter staff can be only gained from:
Overgrown grotto secret (not real yet since this is only a skill suggestion). When you find the secret, a wizard called Con will ask the following text:
Are you sure you would like to learn the elements of the winter?
User: Reply with yes or no.
The following task will be given to the user: Collect winter essence x3, collect winter shard x1.
When collected: return to con by finding the secret again, the following text would appear: Good! Your a great wizard; Then you would unlock wizard staff (Unmastered) Mastered wizard staff would pierce 3 defense. And 1 extra damage for critical.