Use Arctic Assault to escape bad situations, interupt channeled spells or chase runners.
Melee allies can stack Frost for your Permafrost, keep an eye on who they are attacking.
Save Glacial Prison for encounters where you have teammates present to take advantage of it.
Dodging Arctic Assault and Glacial Prison can give you a huge advantage.
Focus on avoiding the second swing of Winter's Wrath which carries the brunt of the damage.
Melee enemies can also stack Frost for Sejuani, be very careful around them.