Mama Mush Slayer Defeat Mama Mush in battle! 0/1

I saw someone with the achievement but when I got to battle Mama Mush its not available. How do you fight her if possible?

While exploring the polar ice fields, (arctic frost) you need to run into the 'monster' and not have the frozen cookie. It's quite the pain in the ass, I haven't gotten past two steps since I've been trying -_-