First off, I'm posting this in the hunter forum simply because I did it as a hunter, though I can assume it's doable by any class with a 40 yard slow. If it needs to be moved, feel free to do so.
As many of you may or may not know, there is a giant purple eel swimming around Shan'ze Dao in the north west of Townlong Steppes. It's called G'nathus and up until 5.2, it was there to scare the pants off of you and not much else (to my knowledge). In 5.2 however, it now drops a companion pet. Now, I have no idea whether it's any good as a battle pet but it looks pretty sweet and if you like collecting pets, it's worth try this as it's really quite simple.
It's a simple strategy.
G'nathus swims around the isle and is rather easy to find, there are smaller eels called "Spawn of G'nathus", but you'll notice the guy/girl when you see it. He/she has 43.6m health and only seems to have two abilities, normal melee hits and an ability called lightning breath which hits for roughly 80-90k.
Now, I didn't bother trying to pet tank as I'd been doing this with a 3 man group previously and he/she was hitting the tank rather hard, so I figured kiting would be the best way.
The lightning breath is outrangeable, so the objective of the entire fight is to stay at max range whilst keeping concussive shot on CD. Standard kiting really. You have the entire island to kite it around and I only ever had one other mob start to hit me, though it just reset after a short time. If at any point you miss a CD and he/she catches up and casts lightning breath, you can deterrence it or choose to take the hit and just heal with spirit bond/glyph of liberation, just remember to slow it again as soon as possible and you'll be fine.
The fight in total took about 11 minutes, but I wasn't going for speed, I just wanted to see if it was kiteable.
It's a lot like whale shark. Stay at the right range and never let the blighter hit you.
I realise this may be a tad convoluted, but it's quite simple and most if not all hunters should be capable of doing this. If anything, do it to see a giant eel floating above the water.
I hope this has been somewhat helpful to anyone interested in trying it. As I said, it's probably doable by any class that can slow from max range and dps on the move.