<<<<<WOTLK AND CATA>>>>
May work on Cata, didn't test!!!
Here it is:
Make a warlock, get Enslave Demon.
Find a demon (preferrably felguard)
Enslave him, he'll have some abilities without CD, for example Uppercut, which strikes and knocks back an enemy.
The exploit is, there's no CD, not even global CD on those spells, it depends only on how fast you click your mouse...
When you've chosen the best spell of your pet, turn off the auto casting of his other spells and make a macro, type
/cast SpellName (where SpellName is the name of your spell)
and repeat this line in the same macro until you reach character limit
Examples:
Felguards at the Blood Furnace can kill last boss easily.
Near thrallmar, there are felguards (this works if you're ally) take one of those, he'll have a skill called Ravage, make a macro as explained above for that skill and go kill everything in thrallmar (yes, you can kill a Wrathful 80 in a sec, unless he kills the demon first)
Note: Do NOT use demons with mana, their abilities will drain the mana quickly so it won't work!
And most of their abilities have some kind of CD.
Note2: Google "Super Duper Macro" AddOn for 3.3.5a (or if the exploit works for cata then for your cata version) it will help you to increase the damage of your pet even further, because it gets rid of the 255 character limit on the Macro. I can kill Naz'grel (boss at Thrallmar, 830k hp) in 2 seconds.
Also, I didn't get a ban for it yet, but I can give a 99% guarantee that if someone reports you the devs will investigate (and possibly temp/perm ban you)
Important Notice!!!
It works with priests' Mind Control! Credits for the priest go to boomkin2k9!
Cata now confirmed too, credits to viralsack for confirming it!