An easy way to level 75 pets to 25 (for the achievement Pro Pet Mob, which gets you the
Singing Cricket) is not finishing the Pandaren Spirit Tamer quest. As long as you don't turn that quest in, you can keep fighting the Pandaren Spirits as often as you like.
Then go to the Flowing Pandaren Spirit and use
Anubisath Idol, a fast anti-critter pet (I use
Clouded Hedgehog,
Golden Civet,
Sifang Otter,
Plains Monitor and the likes) and the pet you want to level. This pet does need to be level 10+ due to AoE attacks (I recommend more then 500 health).
Start with your
Anubisath Idol against his fish. First your
Sandstorm, then
Crush (will miss) then
Reflection when he comes up. Then just finish him with
Sandstorm,
Crush and another
Reflectionif he dives a second time. Try to kill him before he roots you with his
Whirlpool.
Get your fast anti-critter for his critter. If you have
Dive and still two turns left on the
Whirlpool, use it to avoid the damage. Then just try to kill the critter asap. If RNG hates you and your pet dies, use the
Anubisath Idol to finish the critter.
When his last pet comes out, it will do
Geyser and
Whirlpool in the first two rounds, so swap you anti-critter for your leveling pet. Let this pet perform one attack, then switch to
Anubisath Idol. This will be just in time for your pet to use
Reflection and prevent the damage from
Geyser and
Whirlpool, plus do this damage to the Spirit's pet instead. Then just finish this pet off. And boom 4000-5000 XP for your leveling pet

Extra tips: Use your tamer hat.
Reflection will change in 5.2 to Deflection and won't do damage any more. It's still a powerful ability, but killing the enemy will take more time. They might also change the fact that you can continue battling these Spirit Tamers when you don't deliver the quest in 5.2, so best to utilize this asap, just in case. It is horribly boring, so put on some good music or a movie
