One Light in the Darkness (10 player)

Here's what my group did tonite. Overall, this achievement took literally four pulls to do. The only keeper we used was Thorim - he helps to kill the adds in Phase 3, making the fight only a moderately more-difficult version of the normal mode fight.

Group build:

Tanks:
Protection paladin
Feral druid (in dps gear)

Healers:
Disc priest
Resto druid
Resto shaman

DPS:
Blood DK
Unholy DK
Combat rogue
Survival hunter
Shadow priest

Phase 1, we didn't proc any additional clouds to speed up the add spawning. We just let the adds spawn naturally and only had one add (which quickly died) heading into Phase 2. Odds are, if you're trying to do this achievement you already know not to touch the clouds so I won't get into that.

Phase 2, we put a mark over one of the melee dps and over one of the ranged dps - as "focal" points for the raid to group on, as well as focal points for raid dps. The group as a whole stayed fairly tight on top of each other, so that cleanses, heals, brain links, and everything were much easier to manage. The melee group only moved to kill corrupter tentacles so long as they were within cleanse/heal range of the rest of the group - that way if they got brain linked, it wouldn't be hard to negate it. We didn't have either of the tanks "jousting" on the crusher tentacles, but if your group wants to do this, they can.

When it came to the brain phases, we sent the rogue, the two DK's, and the feral druid down. The unholy DK and the feral druid went left (and rotated clockwise) around the brain room, while the blood DK and rogue went right. Killing the adds pretty quickly usually gives you about 35 seconds to reach the brain - and realistally, about 20-25 seconds to dps the brain itself. You already know to kill the brain quickly - we did it in three brain phases. I'd recommend, if you can spare it, to send a hunter down for Aspect of the Pack to speed up the run into the brain room proper.

Phase 3, the entire group clumped together fairly tight on one side of the room. We let the Immortal guardians come to us. The melee killed the guardians while the ranged stayed on Yogg-Saron for the majority of the fight. Be careful with Immortal guardian aggro, though - it can be a tad touchy for the first second or so, and we lost three people to bad Immortal Guardian spawns. We had to use both of our battle rezzes, as well as one ghoul rez, but we still got the kill pretty smoothly.

Just have your ranged dps turn their backs about 1 second before the Gaze cooldown is up, and never have your melee, healers, or tanks face Yogg-Saron. Splash any damage you can on Yogg-Saron as melee dps - this means Consecration, Fan of Knives, Pestilence (plus diseases) and the like.

Overall, this isn't nearly as difficult as Firefighter is. Good luck everyone.

Edit: apparently, some jerk came running through here and downranked every comment made on this achievement so far. Even the ones with pertinent and useful information.

When trying to decide which one Keeper to ask for assistance, it's helpful to know what each Keeper would bring to the table:

Mimiron
[spell=62671]
[spell=65210]

Freya
[spell=62670]
[spell=64169] - Note: This is the only way to regain Sanity during the encounter

Thorim
[spell=62702]
[spell=64171] - Note: This is the only way to completely kill the Immortal Guardians in the last phase

Hodir
[spell=62650]
[spell=64174]

Edit: Wow, is it really that long time ago? I can't even comprehend how much have changed since then. According to wowhead, it seems that you can still complete Glory of the Ulduar Raider Glory of the Ulduar Raider (10 player), but I guess you can roflstomp through the achivements with +2 level 90'ties.

From whats it worth, here is how we did it.

Raid composition:
Retribution paladin
Affliction warlock
Shadow priest
Protection paladin
Feral druid as dps/OT
2 x restoration druids
Restoration/Elemental shaman
2 x rogues

Thorim was our keeper as we believed his ability was key to survive phase 3 (Titanic Storm)
Im not going into details as I assume people know what this fight is about when engaging this archievement.

The first ~10 wipes we realized a few things:
Phase 1
Our 2 trees where capable of healing everything, so our shaman went elemental, which made the encounter more smooth and allowed us to spawn an extra add like we are used to in the beginning.

Phase 2
Yogg + 1 keeper (Thorim) is not as forgiving as 4 keepers since you can't replenish sanity, run slower, do less damage to adds/brain/Yogg and you recieve more damage - RNG plays a big part of your sanity loss.
Although rare, out of numerous tries some of our players got Psychosis 2- 3 times in a row.

As much as possible, we moved in a group and kept 2-3 yards between raid members, this makes it easier if people are Brainlinked, Deathcoiled, get curses etc.
Remember, if someone is Deathcoiled, get away from him because it chains to people nearby. If you use Deadly Boss Mod the person gets a skull above head.
Our protection paladins job was to locate the crusher tentacle every time new one spawned, we didn't find it nessecary to interrupt it. Ranged killed it while whole raid followed him around. Melee killed the constrictors and corruptors in between, constrictors being first priority since they grab people.

Our brain team was retribution paladin, feral druid and the 2 rouges and each of us had assigned our own portal, this is imperative because if you take each others portals, you might not have time to reach another before it despawns. You should aim for atleast 3 times in brain to beat enrage timer so every member of the brain team have to enter.
Personally I used the entrance gate to locate my assigned portal for easy access.
Feral + pally to the left in room and rogues to the right which worked fine for us.

Important! Do not enter brain if you have Curse of Doom unless you have a decurse capable class with you like our feral druid. If you dont, then either wait for a dispell or bubble + cloak of shadows out of it before you enter. Our 2 trees outside couldn't dispell us inside brain and it will be instant death.

Phase 3
We all gathered at our protection paladin while he grabbed adds as they came for us. Shadowpriest and Warlock kept their backs to Yogg all the time and dotted him up while melee were hammering the adds.
I'm the retribution paladin and I realised quickly that our dps on adds was great so I used most of the time judgeingYogg, doing Consecration and Divine Storm to help damage Yogg, this worked nicely since all can damage from behind.

So how did it go and tips
Untill we perfected the above we used 3 hours before the actual kill, the hardest part was getting phase 2 under control and people grabbing each others portals in the chaos.
It worked with 4 dps as brainteam, but maybe 3 dps and 1 healer would do the same trick - its up to you to find out in your own raid.

From a retribution paladin point of view, I did my best to dps but also to support my raid:
Extra adds is really not a problem, but doing this achievement it can be. So we kept melee from adds and only let our ranged deal with the adds, but:
- while we were dodging clouds in phase 1, I judged Judgement of Light for extra healing support
- I used Exorsism and Hammer of Wrath whenever I could on adds in phase 1.
- You can Divine Shield out of Constrictor Tentacle (mages can Ice Block)
- Dont hesitate to use Divine Shield + Divine Sacrifice if you are stuck in a Constrictor Tentacle and noone is near to help you (this never happends ofcourse :D ) or you see raid taking heavy damage.
- Lay on Hands ; use it! whether its on yourself or fellow raider.

GL to all.