Two ways to avoid the Blistering Cold:
Blink (20 yards) and walk for the 5 yards remaining to be safe.
If you are at the beginning of your fight and you have accumulated Unchained Magic stacks. When you are gripped you will be located exactly on the boss continue casting on him and right before the Blistering : Ice block it, you will both avoid the attack and dispel your stacks. And all of that without losing any time for moving. (This is a trick hard to manage, don't do it if you are not confident on your mage)
Unchained Magic:
Do not stack more then 7.
Your can also Iceblock right after getting the debuff it'll remove it and you'll ni longer stack. Keep that option for the exectude phase of the boss (phase 2)
As Arcane: If you have Incanter Absorption you can get a very nice SP buff on this boss. By absorbing frost damages.
In heroic mode, Instability stacks (you get these from casting with Unchained Magic) explode when they run out, so you should make sure you are not near other players when that happens. Try to time them so you do not explode when you are gripped in for Blistering Cold, because everyone else will be stacked on you, making an explosion very deadly. You also take more damage from Instability stacks now, so be careful. Keep your stacks low and your health high!
Your Ice block can be use to remove stacks so you don't take or cause any explosion damage. But prefer removing the unchained magic debuff right away with your iceblock, because even if Instability stacks will not harm you the explosion will still occure and it could be deadly for your nearby allies !
If you are enough confident and you are in the first series of Unchained Magic you can continue your burst, get gripped by Sindragosa, launch a a spell like fire blast (or arcane barrage for arcane mages) during your flying time in order to maintain the debuff, and then you can do the exact same tips as in normal. (be sure to do not have stack as much as it will reach your teammates outside of the Blistering Cold range)