Honestly, her MS is fine. A little lacking, but nothing Sorc Boots (or MS Quints) won't fix.
The patch where they let you pop Flash Frost earlier changed her a lot. At range, you are a great harasser. Double hit with Flash Frost, then follow with a Frost Bite. That's half someone's life. They flee, you farm. If they come back quickly, hit them with it again, they die.
Up close, well, Flash Frost, Frostbite, and Glacial Storm. Just sit on it, poke them a few times with auto attack, and watch them die in two seconds.
If Anivia could chase, she'd be extremely powerful. Right now, the reason her "difficulty" is maxed is because:
--You need to double-hit with Flash Frost.
--She can't chase; you have to Flash Frost stun/Frostbite and pump out damage instead of chasing.
--She can't run; you need to use her egg, and your towers, and your buddies, to make sure that you "survive" fights.
--Glacial Storm needs to be dropped farther from you. Range was buffed recently; you have to make it so they run through GS as long as possible.