That is really strange, it seems to work on mountain after all, so i have no idea what is causing it. I will look into it. Does it happen when you enter from other location into the frozen tundra, like snow city or warp?
The intended way to fight it is to deploy the field effect so its dmg resistance changes and he can take dmg. And yes you should have it, it certainly is not one you get after 15 or higher if i remember correctly. Does it not work? If so i will look into it!
Thanks for the reports!