On this boss your location will be crucial and dependent on how your raid do the tactic, but there is general advice:
Stay away from your mates in order to reduce you chance to be hit by a Death and Decay.
Stay at range to hit both sides of adds and the boss (during the P1).
Don't stay in the future way of your mates, especially the way taken by dps to reach the boss when the Tank brings her at the center of the room. (because they will probably sit on your location location during the P2 and you will be bored by spirits.
Be aware of mind-controlled targets. It can happen that a ranged dps is mind-controlled nearby you, and they can kill you before you have the time to react (Mind-controlled targets have 200% increased damage and healing) [Thanks to Sky for this info]. Keep an eye out and be ready to either polymorph/counterspell, or simply run out of their range.
If you apply this advice you will improve your dps by reducing the number of imperative moves to do.
Remember to focus Fanatics otherwise you will loose a huge amount of dps
In phase 2, if your healers are not doing well remeber you can absorb some frost bolt damages with your frost ward.
Nothing special to notice in heroic mode.
Place yourself in one side of the stairs and focusing dps the adds you have on your side, stay a bit away from your mates to avoid maximum of Death and Decay
If you have Incanter Absorption you can get a very nice SP buff on this boss. By absorbing frost bolts.
Attack Fanatics !
Nothing special in heroic.