Bring in 2 tank healers and 4 raid healers. All 3 of your tanks will be playing an active role here by absorbing Marrowgar's Saber Lash. Basically, your tank and 2 nearest raid members will be splitting 200% of Marrowgar's melee damage between the 3. You may also wish to assign a healer to cover any melee in need of healing.
For positioning, park 2 healers in the center of the room. Place 2 to the left and 2 to the right of Marrowgar's chamber. Try to cover as much as the raid as possible with the healers. There's going to be lots of movement required by the raid. If your raid frames support it, having range finders toggled on to see who is or isn't in range is a big plus.
This worked in the PTR but I'm not sure if any adjustments have been made since. You can park your tank healers directly inside Lord Marrowgar's hit box so that they don't have to worry about Coldflame. The streaks of fire only emanate from just outside his hitbox instead of the center of his character. By standing there, they should be safe. On the other hand, it brings them perilously close to Lord Marrowgar. That's your call to make. It's not difficult to avoid the Coldflames, to be honest.
If you made it this far in raiding into Wrath, dodging fires should be reflexive in nature by now.
How good are your situational awareness skills? You won't have to draw on them as much as you did on Illidari Council (which is the superbowl of not standing in fires). Be prepared to strafe and barrel roll your way from his Coldflames. Keep your head up. You might be able to eat one (8000 frost damage every second for 3 seconds) or so throughout the fight. That luxury won't be there in hard mode so this is a good time to start practicing. (On a side note, I don't know when hard mode is activating. Arthas needs to be killed before that happens.)
Be sufficiently far away from Marrowgar that you give yourself plenty of advanced warning for Coldflames! However, you need to remain close and tight enough to the rest of the raid to keep them alive.
Periodically, your fellow raiders are going to be hit with something called Bone Spike Graveyard. Those players lose 10% of their health per second until the Bone Spikes are broken by the rest of the crew. The longer that process takes, the more you have to do. When I was affected by Bone Spikes, I wasn't able to control my character or cast anything. You are at the complete mercy of the other raiders.
It's possible for Coldflame to affect players stuck on Bone Spike Graveyards as well. Be prepared to blow a cooldown to keep them alive if it comes down to it.
This is the phase Lord Marrowgar turns into a whirling devil.
Bone Storm causes him to whirl around and chase different players at random. Players hit will be nailed for 6000+ damage every 3 seconds until it wears off. A tank can get away with eating this, but most clothies won't be able to.
The reason I would split my healers in such a way is so that there will still be healers available. Bone Spike Graveyard is still present in this phase. It can get scary if all the healers are on one side of the room and Bone Spiked players are on the other side. I learned that the hard way. If you see an opportunity, chip in with some damage spells to accelerate the process. Your raid is going to be in a fairly chaotic state.
Not only that, Coldflames are still going to be hounding the raid. So you're avoiding Marrowgar and fires. Just keep that in mind.
The dangerous part of this phase is when Lord Marrowgar transitions back to phase 1. Why is it dangerous? If your tanks haven't recovered and they're not properly centered on the main tank, then the main tank could end up eating a full Sabre Lash. I actually experienced that a few times on subsequent tries. If you notice that there are no other tanks around to eat the lash, get in range and hit the tank with a defensive cooldown and hope he survives long enough.
Debuffs to add
- Priests: Dwarves might be able to Stoneform off the bleed effect from Bone Storm. I never tried it though. Even if you are raid healing, stay within range of the main tank and immediately hit them with a shield when Lord Marrowgar phases back into his normal stage.
- Paladins: Keep a Divine Shield or Divine Sacrifice ready when your raid needs it. Be prepared to snap to it if several raid members get caught in a Bone Spike with Lord Marrowgar looming near by.
- Shamans: Nothing cool here. Stick with whatever your raid healers ask you to do (which is most likely Chain Healing into the tanks).
- Druids: Anticipate a hit by Lord Marrowgar? Shift into bear to up your survivability.
If you use specific addons to monitor debuffs on players, I recommend adding these:
- Bone Spike Graveyard
- Bone Storm
Go through these phases about 3 times each, and your raid should be home free. It's a fairly simple fight, eh? Don't worry! The next ones get much more interesting!
Want some more advice for working with the healers in your guild? Raid Rx has you covered with all there is to know! Looking for less healer-centric raiding advice? Take a look at our raiding column Ready Check.