Eireen has a knock back ability that most of the raid can joust out of range to miss it from hitting them.
You will notice once inside the circular room that there are three pillars inside here. About 30 seconds into the fight one of the pillars will start on fire. You will need to drag Eireen to this pillar and have her practically go inside the pillar. This is the hardest part of this fight. It will be an art for the tanks to learn just how to get Eireen to touch the pillar properly. You will see a message on screen if you do it properly that says "The frigid aura around the dragon has quenched the pulsing flames, thus hindering the dragon's recovery!"
Doing this slows the rate and dmg at which Eireen casts her AOE's. If you miss one of poles you will soon find this out. Just keep moving Eireen to the next pole when you see it on fire.
Typically the best way it seems to get Eireen to touch the pole is wait for the knockback to hit your tank so it hits your tank against the wall behind the pole or to drag Eireen between the wall and the pole. Just becareful of Line of Sight issues while doing this. Also, you will notice that every 60 seconds or so the tank that has aggro on the dragon will get an incurable stun/stifle on them. This is meant to force you to change tanks periodically so have two tanks at all times working together on placing the dragon so when one gets the stun and can't do anything that you do not let loose the dragon on your raid and hopefully stays on that second tank.
The fight itself is not overly tough from an ability standpoint just be sure that your raid moves opposite of your tank movements so your raid stays on the opposite side of the dragon in the circular room.
Good Luck!
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Challenge Version
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The challenge version of this encounter is done the exact same way as the normal version. The differences found in the encounter are three adds now instead of two as well as a much higher power drain from the elemental aoe. The damage on the AOE's has been increased but not that badly.
The biggest problem guilds will face during this version of the encounter is killing the three adds in time to prevent them from being absorbed and wiping your raid. Three chanters should easily be able to keep your raid with plenty of power.
For whichever tank is gathering the adds, it's important to note that their spawn is staggered by about a second each and that they spawn directly under Eirreen. This makes gathering them a bit trickier than, say, the adds on Imperator Kolskeggr (HM shoulders) who all spawn at once.
Though missing an add (or even two) isn't an automatic fail condition, your raid will be hit with AE damage for each add that is left alive when the next cycle spawns. If you miss one or two and survive, it's imperative to rez anyone who was killed and to catch up on the next set or things will quickly spiral to a full wipe.
Also there is no KB on HM like it is on EM.
Yes they do.
Do the HM Adds have coop strike?
Trying this today. Keeping my fingers crossed!