“Keristrasza was a servant of Alexstrasza’s flight. She planned to stall Malygos’s advance in the Nexus War by eliminating his consort and facing his retribution with a fleet of red dragons. While the plan worked and Malygos was distracted, he used his considerable power to imprison Keristrasza within The Nexus and slowly bind her to his will as a new consort. She is currently being held there and players are tasked with killing her to preserve her freedom.” – WoWWiki
The only thing that you have to worry about this fight is the Intense Cold debuff that is applied when your mage stands still for a period of time. This debuff will both deal damage and increase cast time on your spells. To combat this, you will have to jump in between each spell cast (even instant cast spells) to maintain minimal casting time on your frostbolt, as well as minimal damage.
Keristrasza also casts Crystal Chains, a spell that does Frost damage and roots everyone in place, making them vulnerable to Intense Cold. Fortunately, mages have spells that free themselves out of it.
When Fingers of Frost Procs, you can squeeze in a shatter combo on the second charge before having to jump. For Brain Freeze procs, toss a Fireball in place of a Frostbolt in between jumps, since doing so during a jump will result in the inability to cast a frostbolt as you land, forcing you to jump anyways.
Mirror Image clones will die quickly. Don’t bother using them. Your Water Elemental is immune to Intense Cold, so summon him as soon as the fight starts. Icy Veins as soon as possible as well. After Cold Snap, use your second Icy Veins when Keristrasza enrages, minimizing damage done to the tank. Depending on how long the encounter lasts for a particular group, you might want to save your trinkets for the enrage if you can only use it once during the fight.
Your tools of choice here are Ice Barrier and Frost Ward. If you can keep both up whenever their respective cooldowns are available, you will not take any damage for the duration of the fight. You can Blink, Ice Block, or Escape Artist your way out of chains.
Mage Armor is useful for this fight due to its Frost resist. Use it to decrease damage taken, and to increase the duration of your bubbles in the process.
Gloves of Glistening Runes – Decent stats, but the large amount of Spirit and lack of +Hit signals that this is more suited to priests. The T7 gloves and Ebonweave Gloves have +Hit, and the former has better +Crit.
With the wide variety of tools at your disposal to counteract Keristrasza’s damage, it is certainly possible to take absolutely no damage during the fight (see screenshot below!). With a frost-immune Elemental, this fight is a cakewalk for Frost Mages. Healers will be love you for not having to worry about you, as this is a healer-intensive fight due to having to heal multiple targets pinned down by the chain.