*** Update ***
Apparently during some patch, they started allowing pets to gain experience even after dying. So, no need to get pets up to level 3-4 to start leveling them. Just head right to Frostfire Ridge and dive in with level 1 pets. Now if I could just find the spot I used to level around... It's quite crowded up there now. And at level 65 I was getting 5k exp for each battle. That's not too shabby.
/way 57.65 60.00
Add-ons needed: Rematch, Battle Pet Scripts
Here is my pet leveling team and a TD script to use it. I run this against the Icespine Hatchlings in Frostfire Ridge. It typically takes them out in 7 turns and I can run it at least a dozen times before needing to Revive/Bandage. The only caveat is that your leveling pet needs to be level 3-4 before you start.
1: Rotten Little Helper (1,1,1)
2: Leveling Slot
3: Rotten Little Helper (1,1,1)
For help with scripts, see Xu-Fu's page below.