Did you manage to knock out heroic mode Hagara? Wondering where to go next? The summoner of ooze, Yor'sahj should be next on your list. Perhaps the biggest change in heroic is the extra ooze that spawns that your raid needs to deal with. This means Yor'sahj has more abilities with which to assault your raid. This encounter doesn't exactly have a set script, since Yor'sahj cycles through different slime combinations.
For healing setups, I strongly advise bringing in no more than six healers. Healers will want to configure their UIs so that they can view Deep Corruption debuffs on their raid frames. All in all, Yor'sahj hinges heavily on the ability of your raid to react to the different combinations.
Handling purple ooze
Deep Corruption is the ability to be wary of. If a player receives five heals, they will detonate and deal approximately 50,000 damage to everyone around them. Distribute your raid so that each healer is responsible for their group. If I'm in group 5, I'm taking care of anyone who is taking damage in group 5. I suggest sticking to large bomb heals. Be wary of using AoE healing spells here, since some of them may trigger Deep Corruption stacks. For a full list of spells that activate Deep Corruption, consult this list. Don't go over four applications. Abilities like Divine Hymn will automatically wipe the raid. Abilities like Power Word: Barrier and Rallying Cry are a big boost.
Tank healing is going to be tricky. If you get a chance to use paladins, do it. We assigned the two healers (one on each tank) so that there would be no crossover tank healing (and unwanted detonations).
Of course, if you don't see a purple ooze during a wave, do use Divine Hymn or Tranquility to take advantage of the fact that you're not going to wipe the raid.
Handling blue ooze
Mana voids tank the mana of the entire group. One of the accepted strategies out there involves players using mana cooldowns (Mana Tide totem, Hymn of Hope, etc.) to restore mana to the group and bring the health of the Mana Void down to around 10%. The next time the raid group sees a blue slime, they let their mana drop to zero mana again and then crush the low-health Mana Void, restoring every player's mana. Of course, the raid starts the same cycle again after by beating the newly spawned Mana Void down to 10%.
If you're having a hard time with this, what my group ended up doing is parking a rogue and having them focus down the void while the rest of the raid destroyed one of the incoming oozes.
Tip: If you know your raid is about to pop open a Mana Void, feel free to contribute some additional DPS if raid health is stable. The death of the Mana Void will restore the mana lost when you were DPSing anyone.
Handling black ooze
Yor'sahj will spawn a number of adds called Forgotten Ones. These guys can hurt, and your raid needs to AoE them down as quick as they can before the group fans out to deal with the next wave of slimes. Chances are, your group won't be able to kill all of them in one sitting. The problems I had in dealing with them revolved largely around aggro. Some of these adds will still be alive when your raid is trying to work down another incoming ooze. Make sure there are healers who can cover the bulk of the group. No player should be caught at all on their own.
Handling red ooze
Extremely straightforward. As long as no one's standing outside of melee range, you can simply unload healing as needed. Unlike normal mode, though, more players will be affected by the Searing Blood ability (eight instead of three on 25-man).
Handling green ooze
Players need to stay at least 4 yards away from other players; otherwise, the explosions will hit others nearby. This is up to the rest of your raid. Drill into their heads and get them to stay spaced out or else you're not going to get out of this phase alive. Don't even think about trying to leave green and red up unless you want to force a wipe.
Handling yellow ooze
Pray that your raid doesn't see this guy very often. Abilities are used twice as often, and Yor'sahj's attack speed increases. If you do hit a yellow, though, it's a fantastic time to use a Heroism/Bloodlust to power through that phase.
One of the deadliest combinations your raid group can experience is the dreaded blue/green/purple combination. Not only are you starting off with zero mana, the damage from the green ooze and the storm of dropping ooze from the sky is going to cut down on your ability to stack up for a Mana Void. The limited healing that can be done from purple means that you can't fire out the raid-wide AoE healing that would help. Setting up five points around the area and having each of the five groups spread out around those points helps to ensure that the healers are within range of the rest of their group.
This encounter is going to be random everytime you start it. A little luck will help you for that elusive first kill. The best strategy is to simply be exposed to the encounter repeatedly so that everyone in the raid gets the hang of what they need to do against the different slime combinations.
Need advice on working with the healers in your guild? Raid Rx has you covered. Send your questions about raid healing to firstname.lastname@example.org. For less healer-centric raiding advice, visit Ready Check for advanced tactics and advice for the endgame raider.