So this trick is like the one posted before, glitching your way through the ground to get to C'Thun using a vehicle mount. However with that one, you first needed to defeat the first boss. With this one, you don't even need to do that.
Here is what you do:
Go into AQ40, bypass the first few trash mobs til you are in the room with the Prophet. Now hug the right side wall til you are all the way in the far top right corner, now to your left is some stone you can hop up on. Here you will wedge yourself between the walls and dismount like in this picture:
Now once you are through the wall you should see this. Simply drop down, but make sure to stick to your left or you could fall through the first floor and need to run back up and do this again.
Now once you are down there, match yourself up with this picture and jump off and make sure to land on that lower ledge
At this point hug the inner wall and head left until you are presented with an invisible wall. Once lined up like in the picture, use your vehicle mount to push yourself through it.
Now once you are through the wall, this next part is really easy. Align yourself up like in this picture, and take a jump off:
Now you should be in the hall before C'Thun. Simple head up (not down) and you will find the hall with C'Thun, like so:
Best thing to do is get rid of the vendors, bring 2 passengers, they will be sent through the wall into the exact same spot you are, so you can take down 2 ppl at a time. Have people at the stone outside to summon you back out. Take 2 more. (Yes you can summon level 80s now with the stone). Rinse and repeat. That or bring a warlock to do the summonings.