Tips to Beat Niflheim Winter
You can beat it with a defensive warrior deck, pretty much just stack shields and never attack and win every defense.
Since it doesn’t rely on abilities much, it can handle the extra ability cost without too much problem.
Take the +1 shield per turn trait on level 1, take spiked shield level 2, and melee master level 3.
Use your attacks on riposte, stack defense with wear the shield.
Enemies will have a hard time putting up many cards against you in attacks, and they will kill themselves as they attack into your shields.