If I dual wield frost magic it should make it stronger aswell.
Or maybe use frost magic in the right and shock in the left.
I'll throw in conjuration aswell, two frost atronachs for back up seems wise.
Alchemy is a great tool for any magic user and it fits a witch perfectly.

You can also boost destruction damage threw alchemy as well.
A bit off topic, but now that we mentioned my witch.
I was thinking to go for a dark look to fit the frost magic.
Black hair and black makeup, and black vampire armor. She will wear Skaal armor in colder areas. Seems like a good look to me for this. I already have a character with this kind of appearance, but I am struggling with what build fits her. Maybe a witch is a good idea for her, so I might use that character for this.