10 man regular strategy
Chimaeron is an intense gear check focused around your tanks and healers, while your DPS are kind of spectators through the fight. The mechanics are very simple, but punish mistakes with near instant raid wipes.
The fight revolves around the little robot, Bile-O-Tron 800, and the buff that he provides while active, Finkle's Mixture. This buff causes all of your raid members to become immune to death while above 10,000hp.
In other words, if you are at 20k health, no attack can kill you; a hit for a million will simply reduce your hp to 1. However, once under that threshold of 10k, you are vulnerable. Healers must make sure everyone stays above 10k at all times.
You activate the robot before the pull by talking to the gnome NPC in the cage, Finkle Einhorn, who you last saw crawling out of the corpse of The Beast many years ago.
The tanks must be ready to taunt the boss back and forth to avoid instant gibs, and the offtank must have the effective hit points and mitigation to survive two hits within a fraction of a second.
This boss cycles through a "spread out" phase and a "stack up" phase before reaching a burn phase at 20%, at which time all healing is shut off and the raid, including healers, must dps him down as fast as possible.
Caustic Slime — A "meteor" style spell that hits a random target other than the tank for massive damage, split between all nearby raid members, and reduces their chance to hit by 75% for 3 seconds. If everyone stacks for this spell, the raid dps will drop dramatically. Instead, raid members will want to get hit with this spell solo, since Finkle's Mixture will save them from death.
Massacre — An AOE that hits the entire raid for 1,000,000 damage. Not a typo! This is survivable, again, because of Finkle's Mixture. Make sure the entire raid is about 10k health before this spell finishes, as well as immediately after. DPS will want to use any self healing abilities they have to assist the healers with this difficult task.
Break - Stacking debuff that causes the tank to take 25% increased physical damage per stack, up to four stacks. After a couple stacks, a single melee hit from this boss does 250k+. With 4 stacks, expect hits in the 400k range. Healers will need to get the main tank over 10k health in between each melee attack. Fortunately, the boss does not attack very fast.
Double Attack — On his next attack, Chimaeron hits twice within a window of .2 to .4 seconds. If the first hit takes the tank below 10k health, the second is instant death. This ability is unsurvivable with even one stack of break, so make sure your offtank takes every hit and doesn't get the break debuff at all.
"Spread Out" phase
While the bot is up, and everyone has Finkle's Mixture, the raid must spread out. This is most of the fight. Spreading is necessary so the Caustic Slime hit debuff doesn't hit every single person at once.
"Stack Up" phase
The boss will knock the bot offline, and the two heads will start "feuding" and attacking each other. During this short duration, the Finkle's Mixture buff is lost. Any caustic slime casts will instantly kill any raid member who is out alone.
For that reason, the raid must stack up somewhere, preferably on the tank. Fortunately, during this phase, the boss does not use his melee attack, since it would be instant death without the bot's buff.
Once the feud finishes casting, the bot comes back online and the fight reverts to the "spread out" phase. This cycle continues until 20%.
"Burn him down!" phase
At 20%, Chimaeron enrages and casts Mortality on the raid. This shuts down all healing completely, meaning that healers need to start dpsing the boss. He will also become untauntable, preventing taunt cycling cheese, and will take 20% extra damage until either he or the raid dies.
During this phase, he will also stop casting any of his spells besides Double Attack. The tanks will almost certainly die here, but the key is to kill him before he kills you.
Organize your raid so that everyone is spread out as much as possible around the boss. Try not to go too heavy on melee. Have a Death Knight offtank, if at all possible. If you can't get a DK, pick the tank with the best effective hit points for OT duty.
Have one healer assigned to saving people hit by Caustic Slime, and the other two will keep the tanks alive. Keep vent clear of chatter so that you or someone else can call out each double attack, massacre, and phase changes. Save lust for 20%.
You are the MVP of the raid. You have only one job: taunting the boss when he casts Double Attack. As soon as you see the icon, which looks like Devour from Warcraft 3, taunt immediately. You should, hopefully, survive the attack.
If you keep getting instagibbed by double attack, look at recount death logs to figure out how much damage he is doing. You might need to reforge/regem for stam and armor to survive the double attack, as avoidance does not help at all here.
After the offtank taunts, watch his health. He will get walloped, and that's your cue to taunt the boss back. You can also just watch for the double attack debuff to wear off; after it disappears, wait about one second and then taunt (to make sure you don't accidentally eat the second hit).
You have a very difficult job. Keep everyone over 10k. During the stack up phase, major raid heals are needed to survive the caustic slime (meteor) attack. During the spread out phase, 10k hp = 100%, so turn of HP percentages if you have it on.
The offtank, however, will need to be at full HP at all times so they can survive double attack. If you have an absorb shield, try to use it right before the offtank gets the boss.
You have the easiest job of all. Just follow instructions to spread out or stack up. There are no fires to avoid or any of that, just spreading and stacking. Save your cooldowns for 20%. Try to help your healers by using your own recovery abilities (especially classes with a heal spell) in tough situations.
If you are stuck at <10k health and a massacre it about to hit, pop a healthstone or potion. Anti-death abilities such as cheat death, cauterize, iceblock, pally bubble, and so on are a huge help here.