I agree that blizzard is too powerfull, especialy considering it's low mana cost.
It's really too easy to conquere entire cities with only 1 spell that cost 8 manas (with the mantle of ocean and various mana reduction spells / objects) : It's zone of effect is perfect for that.
And I've never understood why the more ennemies are on the maps, the deadliest it is. What is this logic ?
I understand that the more water shards you own, the deadliest it is, but why the more ennemies ?
Having said that, it's clearly my favourite weapon... so deadly !... but I'd say, too deadly to be fair.