I'm not sure if it's THE fastest way but I always take advantage of the 3 Flying team from the level 3 Pet Menagerie (Jahann, Samm, and Archimedes) to easily power level my lower level pets.
What you need? A level 25 Nexus Whelpling (Or other pet with the Mana Surge ability)
Basically when the fight starts, you throw out your low level pet that you want to power level and let it get hit by Jahann when he uses Arcane Storm. (As long as your pet has like 180-300 hp, it should survive)
After it gets hit, switch in your Nexus Whelpling and use Mana Surge. He'll kill both Jahann and Samm with probably 300-500hp left going into the final battle with Archimedes. Get off 1 Tail Sweep or Frost Breath and Archimedes will have like 300 HP left.
Throw in some other random level 25 pet to finish Archimedes off and you'll have gained 2,000-2,600 xp in very short order for your low level pet.
The ease of this fight, coupled with being able to instantly heal your pets using the NPC 2 feet away, and you could bunker in your Garrison for a while and level an enormous amount of pets both quickly and efficiently.