20% reduced damage seems like a lot. Masteries contribute to damage reduction too. I just started a post about her, I suggested something where her passive E would stack instead of just increase the duration, and then have her E scale off of how many stacks of permafrost the enemy has on them. That would make things a bit more interesting.
You said early game though, I think her Q range needs to be increased, for one. Maybe change it all together: have it charge up, and make the impact point 2 areas. The smaller inner circle would root or stun enemies, and the outer ring would slow them based on how far away she charged from. I have a post about it already, these are some of the ideas though.