Hello! Been awhile since I contributed. Incoming my nice photo montage of this exploit/exploaration.
Step 1. Start here, in the Mosh'Ogg Ogre Mound Cave, pft fancy smacy name.
Step 2. Position yourself at the red circle inside the cave and start doing the mount bug. For those who don't know what that is: Basically you start mounting and when the cast is almost finished you jump.
Step 3. When you've done the mount bug and jumped to the end of that wooden ledge, mount up on your flying mount and then just fly straight down.
Step 4. Now you're basically done the whole ''Get under Eastern Kingdoms''-thing. Start flying towards Gurubashi Arena. Fly kinda low and watch out for ''The Sundering'' don't fly into that.
Step 5. You're now under gurubashi arena, start flying just up regularly untill you hit the ground, like in this picture. And then just fly up on the ground, you're now inside the walls/the structure of the arena.
Step 6. When you're on the ground, just walk up to the ''room'' or what to call it and just go inside it.
Step 7. There you go!You can't go out the wooden fench but you can go back the way you came from.
Just tell me if you need more explanation and I tried to describe everything as much as I can!
Best regards.