By the time you're 23, your Blizzard should be pretty handy for group mob pulls. Improved Blizzard will give Blizzard a chill effect and lower the targets' movement speed. Permafrost will increase the duration of said chill effect and slow the mobs a little more. Frostbite will give your Blizzard (now with chill effect) a chance to freeze the target in place. And Ice Shards can double your Frost damage.

So basically you can run into a small group of mobs, get them to chase you, Frost Nova them all in place, turn around and Blizzard them to death before they can even touch you. You'll have to be careful of mobs that can hit you with range attacks though because that will decrease your channeling time. So a good policy is to cast a shield spell like Mana Shield (and/or Ice Barrier at later levels) just before you pull or just before you begin casting Blizzard to absorb the damage and prevent loss of channeling time (you can also pick up Icy Veins which eliminates loss of channeling time due to damage, but it has a 3-minute cooldown and is one more thing you'd have to cast).

Of course, a Blizzard focused style of play will eat up your mana bar quickly, so you'll be drinking a lot between fights.