Chimaeron 25 Man

This will be the first tactic I write from downing a boss on 25 man (although I posted a 25 man tactic yesterday, that post was initally written as a 10 man tactic and I added the 25 man parts afterwards). There is no special reason for this, only perhaps that our 10 man groups have been progressing faster than our 25 man group. Note that there is no difference to the tactics whether you are 10 or 25, beside the obvious - you are more people and Chimaeron has more hp to counter that. Some skills deal more damage too since there will be more healers to deal with it. But the tactics are exactly the same.
Chimaeron is one of those fights that require minute preparation and execution to be able to pull it off. It's probably as far from a fight where you can "wing it" half the fight, as possible. Although we have seen this pattern for many Cata raid bosses, I couldn't say it is more true than when fighting Chimaeron. If you don't know everything about his abilities, you will wipe and you will wipe fast. As most other bosses, Chimaeron has two different phases of which the first one is a "handle the shit"-phase and the last one is a "nuke him down" phase.

Preparation
When you start the fight you will get a buff called Finkle's Mixture through the Bile-O-Tron. As long as this buff is active, no skill or attack done to you while you are above 10k health will be able to be a killing blow. The first phase revolves around this fact and all healing will have to be done with this in mind. This fight is a healer fight. Whether you have good or bad healers will make all the difference here, and they will all have to pull their weight for you to be able to pull this off. We used 7 healers and assigned one healer to each group and the remaining two to the tanks. Sticking to your assignment has never been more important, because you will never know when you'll need that gcd to save someone. As a healer you will just have to trust your fellow healers in that they are doing their job, dpsers should of course help out whenever they can. In any case, make sure you have covered who is healing who in greatest detail and make sure people stick to it. Everyone will have to spread out 6 yards apart and you should have the position of everyone in the room set before starting the fight. Everyone will have to remember where their position is and make sure to get there after the aoe.

Phase 1 - 100-20%
Keep complete focus on your healing and make damn sure not to overheal, and this phase won't be so horrible. Because of the Bile-O-Tron, you don't have to keep anyone at more than 10k health as long as it is active, with the only exception being the off tank. When Chimaeron gets to 20%, he will enter phase 2.

Caustic Slime - The reason you should stand 6 yards apart is because of Caustic Slime. It is thrown at 4 random players for 280.000 damage (10 man) and approximately 400.000 damage on 25 man (don't have exact numbers). Because of Finkle's Mixture this won't kill you, but bring the affected players to 1 health. These players need to be healed above 10k hp asap, so that they will survive another throw of the Caustic Slime (if they are unlucky enough to get it twice in a row) and most importantly, to survive the Massacre (explained below).
Massacre - All players take maximum damage. This means the entire raid will be at 1 hp, and need to be healed above 10k asap to survive the next Caustic Slime.
Feud - Chimaeron will stop his melee attacks for 15 (?) seconds, giving you time to heal everyone up to max health to deal with the massacre when the Bile-O-Tron is inactive.
Double Attack - A really annoying buff on Chimaeron which will have him strike twice on his next melee attack. The offtank needs to taunt asap and be at full health to be able to take this blow.
Break - A debuff placed on the current tank that will increase the damage taken by 25% and healing taken by 15% per stack.

How to handle it;

1. Have everyone spread out 6 yards apart. 
2. The tanks will be taking the most damage in this phase, except during the aoe. Make sure everyone knows what to heal and that everyone has an assigned healer. 
3. Stack Break on Tank A, as soon as Chimaeron casts Double Attack, have tank B taunt to take the blows. Tank A will taunt back asap (but after the blows of course) to make sure that Tank B doesn't get any Break debuffs, so that he will always be able to take the Double Attack when needed. For this reason Tank B has to be topped off at all times. 
4. Deal with anyone who is affected by Caustic Slime by healing them above 10k. Healing them anything above this is a waste of mana! 
5. As soon as Chimaeron casts Feud, have the entire raid stack up on a designated target and start spamming (yes spamming) your aoe heals to top the entire raid off. Use any aoe heals necessary, but don't waste mana of course. Chimaeron will still be casting Caustic Slime, and you have to be stacked to spread the damage to be able to survive it (remember it does several hundred thousand damage per slime, spread amongst the targets). 
6. As soon as the Feud is over, spread out again. Make sure everyone are keeping their range to eachother. 
7. Rinse and repeat until 20%.

Phase 2 - 20-0%
Healers really get to relax here when compared to the first phase. When Chimaeron gets to 20% he will cast Mortality, which will reduce all healing done by 99%, meaning it will be completely useless to throw any heals whatsoever. Therefore, all healers should instead start dpsing in any way they can. He will also take 10% more damage, so this is a burn phase if ever. The Bile-O-Tron will be active so everyone in the raid can take two blows from Chimaeron before they die. He will also do his Double Attack, which will one shot the target. You basically have two melee strikes times the amount of people in the raid and any time you can gather from kiting him on you to burn him down before you wipe. People will die during this phase, but all cooldowns should be used and anyone who becomes the target of Chimaeron during this phase should try to kite him as much as possible, buying time for the rest of the raid to burn him down.

Priest specifics
This fight was iffy because I felt like I wanted to be disc to handle the Caustic Slime and the burn phase, and holy to handle the horrible aoe damage during Feud. This is one of very few fights in Cata so far where I did go very oom towards the last phase, because the aoe healing is pretty intense. It is imperative that you keep track of the hp of your assigned targets and don't heal a drop above the amount necessary, 10k, to save as much mana as possible. Having an UI that will help you with this is recommended. Remember that any heal above this amount is both wasted mana and a wasted gcd, since it won't do any difference whatsoever if the target is at 10001 hp or 90000 hp. I liked this fight actually, it gave a fresh take on healing, really putting emphasis on the Cata way to deal with things. Maybe a good way to get your raid healers to learn how to conserve casts and mana? Chimaeron isn't a fight with alot of movement, unless you happen to be positioned far away from the stack target during Feud. Therefore I recommend Inner Fire. Try to switch to Chakra: Sanctuary before the Feud, to get as much aoe healing throughput as possible. I think Disc could handle this fight well, as long as there is someone else to pick up the aoe-healing somewhat. You could use the bubble during one or two Caustic Slimes when the Feud is up, but it won't compensate the Lightwell, CoH and Sanctuary. If you got a choice between the two I recommend Holy (when writing this, before the disc buff 4.0.6 patch). Alas, aoe healing is what cata raids are about, and aoe healing is what holy priests do best.