Phase 1 (100% - 80%)

First off, the area around Halion will be surrounded by a ring of fire cutting everyone in, and out. So the should move in together for the pull.

Secondly, he's a standard dragon. You don't want to be in front, there's Fire Breath and cleaves. You don't want to be behind him either due to the tail swipes.

Tank her near the wall of fire, this would allow the most room for the rest of the raid to work with.

Fiery Combustion

This would be the major ability of this phase. What it does is, a random player gets a debuff. It is a DoT that can be cleansed or decursed off, and when it does, or when it expires naturally, drops an area of fiery hurt around them (big bright yellowy circle patch), Combustion, which also knocks back players around it. The size of that patch is proportional to how long you took to cleanse it off, the longer, the bigger.

So you might want to think of this like a hybrid of Necrotic Plague and Defile. You don't really want to instantly cleanse it, or it leaves a big fiery puddle in the middle of the raid. And you can't ignore it or you'll leave a big fiery puddle under the entire raid. You want to give the player like 2-3 sec to run to your designated spot, probably along the edges of the wall, then cleanse.

Meteor Strike

This is the other major ability of the fight. The ground will look like its Hellfiring, then the Meteor will come and basically kill everyone still there. Also, shooting out lines of fire to not stand in.

Pretty simple, don't stand under fire.

Heroic: These will also spawn fire elementals, one big one to be off tanked, and a bunch of small ones to be aoe'd down.

Phase 2 (75-50%)

Halion shifts into the Twilight Realm and a portal opens up for the raid to go through to face him on the other side. Leave a tank and a healer behind if you want for quicker pickup in the next phase, while the rest of the raid goes in.

In here, the raid will start taking some shadow damage (DuskShroud). And standard dragon positioning procedures remain. Tank it just like Phase 1.

Soul Consumption

This is exactly like Fiery Consumption in Phase 1, except in shadow form, and it sucks players into it (think Gravity Bomb). Do the same thing, run to the wall as quickly as you can, wait 2-3 seconds before the cleanse giving him that chance.

To repeat what this ability does, it leaves a DoT, which can be cleansed or decursed. When that happens, or it expires naturally, it leaves a giant puddle of nasty behind to not stand in. The longer you take to cleanse, the larger that puddle will be.

Twilight Cutter

This is the other major ability in this phase, and it's not a shadow based Meteor. Instead, there'll be these 2 big purple ballz. There'll be a big beam connecting them. It will rotate around the room. Those that touch the beam, die.

So think of it as another C'thun Eye Beam, Lurker Water Spit, or any of those things. Sweeps the room, don't get hit. Tank would be rotating the boss along the beam to avoid it himself.

Heroic: There will be 4 orbs, making your rotation a bit more tighter.

Phase 3 (50-0%)

Halion will start attacking in both realms at the same time. The portals are open up permanently so split the raid up and send half back into the Physical realm.

Ideally, send your ranged dps in the Physical realm, and melee dps kept in the Twilight realm. The Cutter will be easy for the melee to rotate around while the fire easier for ranged to simply move out of.

Corporality

What this is, each time you do damage in one realm to him, he gets pushed into the other realm. Both sides start at 50%, so say you do more damage to him in the Physical Realm, he starts being 40% there, while 60% in the Twilight Realm.

But, the more in one Realm he is, the more damage he does, and more damage he takes. The important part is, he does more damage, so one shotting tanks are imminent. You want to maintain a balance of dps in both realms to keep it in a 40-60% range.

Also note, if you simply stop dps on one realm, Halion will start to rapidly regen health, so that's not an option. You must keep dps on him in both realms, at all times, so just have people take the portals to balance out the dps when need be.

That's about it to the fight, good luck!