A few more tips, mainly mage related.
As Darieth mentioned, use the jump to reset stacks technique whenever you can. This is easy to set up for the start of the fight. Not so easy after. You want to try and time an instant cast spell when you are jumping so that you do not lose out on dps. Being a fire mage this is quite easy. Either I do it when I refresh my bomb, get a proc and use fire blast/inferno blast, Pyroblast!, or one of my shields.
Make sure to clear your stacks before each Total Annihilation. This is imperative. Your healers will oom way to quickly otherwise. If you have any self-healing or shields then use them for this.
You want to get the adds in p1 down to about 700k before pulling them out of the inner ring. This seemed to be our sweet spot. It gives the tank enough time to drag him out before you kill it. But be careful. Last thing you want is for the add to either die within the inner ring which will give it the damage increase, or for it to go out too soon (above 25%) and have the 90% damage reduction on it.
The sparks in p2 are another real dps race. You want to spend as much time between sparks hitting the boss. If you get procs then it tends to be best to keep these for the sparks. You want to be getting both a Pyroblast! and a heating up set up for each spark. If you manage to get both, then they will die quickly. Without a Pyroblast!, it's going to be harder, especially when you get to the third and more waves. Keep Combustion for either the forth or fifth wave as you will need your really big hitters at that stage due to the speed of the sparks. Remember that it's not how many waves you kill, but how many spawn that matter. eg If you kill 3 waves the boss will get 4 stacks.
When the sparks overwhelm you, keep hitting the boss until the pillars are targetable. Also utilise the inner circle 50% damage buff as much as possible. Just make sure to get off before it disappears