This is my rough strategy guide, taken from experience and various videos, Please feel free to correct me + add suggestions and comments i case I missed something.
this is only for the 10 man version, though It can probably be adapted easily for the 25 man version.
I didn't include the mini bosses, because they're boring and this took long enough to write.
My recommended Raid makeup of characters with roughly the same gear level is 6 DPS, 2 healers, and two tanks. 6 dps to keep DPS roughly even between realms, 2 healers, to keep heals up on both realms, and 2 tanks, because Halion will need to be tanked in each realm.
In the first phase in the Halion fight, the tank should have the boss near the edge of a circle, so there is room for the ranged and healers to spread out and to avoid it's abilities.
As usual with most dragon encounters , Halion CLEAVES, TAIL SWIPES and BREATHES FIRE. so stay away from the head and the tail.
There are a few abilities used in the first phase
The First is an ability called Fiery Combustion this ability is a stacking de-buff which deals 4000 fire damage every 2 seconds and stacks a de-buff called Combustion. when this is dispelled, it leaves a patch of fire on the ground which slows movement speed, and causes damage. The size of this patch of fire is directly relative to the amount of stacks of the de-buff the player with the affliction has accumulated, so the player must instantly move away from the raid, and as soon as they are a reasonable distance away, the player must be dispelled, and then move away from that area again.
It's important you don't dispel too early or too late. There should be a dedicated dispeller, and it should be communicated
(can be dispelled by any kind of dispeller, including paladins cleanse, Druids, and Warlocks felpuppy's Devour magic)
Second ability is an animation under a random player's feet that looks like a warlocks hellfire, you need to strafe left or right when you see this ability, as after a few seconds, a very large red orb of fire will come down and then explode in a very large figure X (possibly figure 8?) like pattern of fire stemming out from that central point. It's very big, and should be very easy to avoid. Just don't stand in fire. =P
Keep going until you reach 75% Health
Halion will shift into the Twilight realm. Everyone is to take the portal that appears and is to spread out again inside the area. and Halion should be tanked sideways to the raid to avoid the cleave, tail whip, and the flame breath. Bloodlust is best used at this point as this will burn the phase between 75% and 50% as quickly as possible and you can then move on to the third phase.
there will be two rotating orbs going around a large circle around you, every 30 seconds or so, and will last for 10 seconds, a beam will appear between them. do NOT stand in the way of this! it's Instagimp. keep moving around so you aren't between them at any point.
(the trick in this phase is to try to rotate Halion in such a way that the tank does not get pwned by the Orbs Twilight cutter when it activates, and also to remain as "side on" to the raid group as the tank can manage.)
there will be a replica ability of Fiery Combustion in this realm, called: Soul Consumption, and it should be dealt with in exactly the same way as it is in the physical realm.
Keep going until 50% health.
The raid will need to divide into two 5 man groups. We used 1 tank, 3 DPS and 1 Healer in each. one in each of these groups must be a dedicated dispeller to deal with the combustion/consumption.
Remember, because of the meteor effect, it is easier to move around as RANGED in the physical realm, and easier as melee to move around in the twilight realm, so I suggest dividing your raid up that way.
Portals will appear in each realm, and Half your raid will need to emerge from the twilight realm into the Physical realm. There will be a version of Halion in each realm, each one will need to be tanked independently.
You will see text on your screen on this phase, called "Corperability" this is a way of balancing DPS in between realms. The Percentage will start at 50%,and will move up or down depending on how much the dps in each realm is dealing. you should aim to keep the Percentage between 40% and 60% in this phase. Communication is very important in this phase, in order to ensure that the DPS in each realm deals roughly the same damage, and to ensure that the DPS in one realm slows down when they need to. Raiders tend to tunnel vision in this phase because the boss is getting towards low health, and they get excited, so make sure the raid leader keeps the raid updated in vent constantly through this phase as to how the corperabilty is going, and knows how to direct the raid in what to do. this is a precision phase, not a zerg phase.
Remember, the abilities from Phase 1 in the physical realm, as well as phase 2 in the twilight realm, also occur in this third phase!
Good luck! Grats on your realm first kill! =)
That's all I have on this fight. Please feel free to correct me or add suggestions as a reply, I welcome the input. But remember, This is my first guide, so Play nice.
Please remember that this info was taken from the PTR, so it might be different. please take that into account.