It's been a while since I had one of these posts, and yes, I know, most of you have killed Halion quite a few times by now. This is the type of fight that the mechanics are fairly straightforward, but there are a few tricks I can mention that might be helpful for maximizing dps nonetheless.
This is likely to be my final Boss review for DPS DKs prior to Cataclysm. I don't plan to discuss the three mini-bosses preceeding Halion, as they were puggable on day 1, and although you can still get ranked on World of Logs (I've been ranked on General Z a number of times), these fights aren't true boss fights.
Halion is is a fairly simple portal-type fight, in which your dps team is split in half for much of the fight. For most groups dps is not the primary concern - especially on normal difficulty - so your priorities on this fight actually are: 1) Don't die, 2) Balance dps in both realms, and 3) Do as much dps as you can given 1) and 2).
So, here we go!
Pull/Phase One: Tank runs in from just outside the circle, often perpendicular to the raid for optimal boss positioning.
As usual, it's helpful if your tank gives a decent count before the pull so you can summon your army. As most of you know I like to summon just in time to pop haste potion just before entering combat. Then, of course, run in and start hitting the boss.
It doesn't matter whether you're unholy or frost for this fight. For the most part there are no range issues to consider, and for heroic both specs do well on adds.
So, for phase one, you should simply dps your merry heart out. Depending on when your raid does heroism/bloodlust, you might want to save your pet cooldown. My raid heroes at start of P2, so some prefer to save their gargoyle/ghoul cd. Even with heroism in P2, however, I use my gargoyle at pull, because the cd is usually up again in P3 if I use it early. Two gargoyles without heroism > 1 gargoyle with heroism. I usually pop my second haste potion during P2 heroism.
During Phase 1 you'll be standing to one side of Halion hitting him with everything you can, and the only thing to watch out for is the Mark of Combustion; obviously, if you get it run to the edge of the circle until you are cleansed, then run back in. The flame trails after Meteor strike are actually very small and you can stand almost on top of them without taking damage. If you happen to have a few empty cooldowns, pop anti magic shield, stand on a flame, and death coil or frost strike away. Just make sure to stay in melee range when doing so.
Phase Two: Take the portal and start dpsing Halion in the shadow realm, but be sure to let the tank get agro first. We pop heroism here because we want to get through the phase as quickly as possible. The main ability to watch out for is the Twilight Cutter. On normal, you can survive getting hit with it, particularly with AMS up. On heroic, you'll usually die. Obviously, keep an eye on the orbs so you know where it will come from. There are two ways to do this effectively. One is to have the tank constantly turning the boss with the orbs, and the other is to have the tank stay still between Cutters, and only move when a cutter is about to start. On heroic, to be safe we just move the boss constantly. In any case, what I like to do is move as far to Halion's front left shoulder as possible without getting cleaved, stop, and dps. We can dps on the run fairly well, but, still we are at our best standing still. I stay in that spot doing dps until Halion has turned far enough that I'm on his back left hindquarter, just before I'm in tail swipe range. Then, I strafe to his front left and start over.
Strafe - DPS - Strafe - DPS.
During cutters, I try to stay pretty much in the middle of the cutters just to be safe (on normal there is more room to continue the strafe-dps-strafe-dps strategy if you want).
Drop Soul Consumption as far to the front left as you can, so you minimize running out time while still doing some dps.
Phase Three: Actually, DKs should almost always stay in the shadow realm, since melee are much less affected by the movement than ranged. If your raid is very melee heavy, you may be asked to move back to the physical realm. But for the most part, you'll be in shadow, where you'll continue the same mechanics as P2. The only wrinkle of course is you may need to slow dps is the physical realm falls behind. No additional tricks for more dps here, I'm afraid. If you popped your gargy early, however, you should have another chance to use it in P3.
DPS for DKs should be solid on this fight. There aren't any major dps buffs, but for the most part you should be able to stay in melee range and keep hitting the boss.