Ok so this is not a game breaking exploit but it can help.
If you are a hunter going in portals @ Yogg-Saron in phase 2 and you still have your pet with you (it didn't die), when you get out of the brain room your pet remains there dps-ing the brain (without geting MC ofc) and if you keep a low range from Yogg (outside) it will not despawn. But if you move to far out from the boss for a sanity well for example it will despawn and you will have to call him.