Spec : Presence of Mind, Ice Barrier, Nether Tempest, Invocation.
Glyphs : Combustion, Evocation, Iceblock
Gear : You will need a Mastery build to boost Ignite and Combustion. Arcane reforges are great. Note that I did not have to re-gem, reforges are enough, especially if you have a better gear than I do. I am ilvl 484 with 2pT14, i'd say it's the absolute minimum.
Buffs : Play with mage armor. +300 int food, +1000 int flask, Drums, pre-pot.
Fight : The fight does not differ that much from Normal mode. The cleave parts are replaces by Firestorms, and you get Alysra's Razor faster, hence the Mastery build. Be really careful about Firestorms : your biggest enemy is not the damage (you don't even need a shield, it does about 250k dmg) but Wings of Flame are : be sure to refresh it ! You will need 20 seconds or you will fall to your death, and the fight is so buggy sometimes I had 2 circles in a row that did not refreshed the buff ! So double-check, as I had a couple of close calls during the fight.
I'm still using the Alter Time trick, it's not mandatory if you have a good gear.
A note about DPS : start a normal rotation with Fireball and as soon as you reach 15 stacks of Blazing Power just spam Pyroblast. Use Heroism for the 1st and 3rd Alysra's Razor (1M DPS, wheeeee!). Also, pay special attention to Combustion, especially if you have a low gear level. You have to use your Combustion 3 times in each phase. The first time as soon as you can when P1 restarts, the second time less than 10 seconds after you reach 25 stacks and the third time during tornado phase. Remember that if you apply combustion later, some ticks will benefit from her +50% damage taken debuff. Removing 1 or 2% while you can't hit her is good