Well, there might be a lot of ways to level your freshly tamed hunterpet. The way I prefer is to kill the Converted Heroes in Icecrown.
They ususally spawn in packs of 4-8 mobs (as everybody knows already) and can easily be killed with 3-4 multishots (SV spec). It takes around 5 packs of heroes to gain one level-up, so you should be able to close the 3-level-gap extremely fast.
Well, that's how I close the 77-80 gap. How do you level your new tamed pets? You got a faster way?