C'thun is dead! Arguably the illegitimate way. I didn't Fraps it because my PC isn't the best and I was afraid the lower framerate would reduce my chances. But I have screenshots:
http://i.imgur.com/RoqRh.jpg
http://i.imgur.com/Ae9wU.jpg
Tactics:
Phase 0:
Before the fight, you will need to pull Ouro up to C'thun's room. No problem here, only a slight note that you'll want to dismount as it is possible for him to reset if you go too far away. Just kill any Scarabs that reach you.
Phase 1:
Just kill him. You might need to be careful of his eyebeam if your DPS isn't high enough, but that's simple as cake.
Phase 2:
Your goal is simple, but made as difficult as possible. You need to kill the two Flesh Tentacles inside of his stomach, then defeat C'thun while he is Weakened. To achieve this, you need to be Swallowed; however, being Swallowed makes C'thun instantly kill you after a few seconds due to his wipe mechanics. Therefore we have Ouro to teleport us out. Sadly this means that the majority of the time, we only spend a fraction of a second in the stomach, and simply have to pray for enough time to hit a Flesh Tentacle.
Make sure to have a macro ready along the lines of:
/target Flesh Tentacle
/cast Arcane Shot
Therefore, the real challenge in phase 2 is getting swallowed as often as possible. For this, you need to dispel Stomach Acid, by running into the center of C'thun's hitbox. You'll get knocked back, and soon enough Swallowed again for another chance.
To make matters worse, you'll be taking constant damage from Avid, Mind Flays, Ouro's scarabs, and big tentacles. Make sure to dispel any Hamstrings you're afflicted with and AoE down any Scarabs, but otherwise I'd advise to ignore any other adds and focus purely on dispelling as often as possible. The spawn rate of the tentacles increases as the fight drags on, and they will eventually kill either you or your pet, if you don't get him down fast enough.
Once you've got a lucky break and kill both tentacles, simply pop all cooldowns and get him down. You will NOT get a second chance, so make it count!
Side note: Legitimate kills of C'thun are impossible. There is no situation in which C'thun will not kill you inside the stomach. You cannot reach the teleport pad fast enough, no abilities will save you, and neither will managing to kill the Flesh Tentacles. You cannot prevent being swallowed, either.
Side note 2: I've noticed my World MS increasing substantially after attempting these C'thun kills. I'm thinking the exploit puts some kind of strain on the server somehow, so I'd advise not to power too many attempts in a row, give it a soft reset after a few wipes. It's in your own interest as you don't want the instance to freeze on you just as you manage to weaken C'thun, like happened to me >.<