Requirements:
- level 25 Lil' XT or Landros Lil' XT (Abilities to choose: Zap, Heart Broken, Tympanic Tantrum)
- the pet you wish to level (anything from 1-24)
- and a supplementary highish level pet for those times when EVERYTHING simply misses and RNG doesn't go to your favour (any pet from 15+)
Process:
Put the pet you wish to level in your first slot.
Head on over to The Grummle Bazaar in Kun-Lai Summit (64, 61). It's the same place you buy the Grand Expedition Yak.
There is a stable master here, make sure your pets are fully healed.
Fly around a little and find a pet to battle, shouldn't take too long, they're everywhere in this area
Use an ability on your low level pet so that it can be granted EXP. Switch it for the Lil' XT (If the pet is level 1 there is a high chance it will die, just flee, use the stable master and try again)
With the Lil' XT use Heartbroken. If it goes well, you will take some damage and Heartbroken will be triggered, if you happen to dodge or the enemy misses, that's why you have the supp.
You next move will be Tympanic Tantrum. Over the course of the 3 hits it does, it will likely kill the first 2 pets, or all 3 at once. Lil' XT could die during this, but mechanical pets revive for 20% HP!
Because the majority of pets out here are beasts, mechanical pets are super effective against them.
If enemy pets are still alive, finish them off with Zap.
Recover HP at the stable master and repeat.
What makes this special is that if you do kill all 3 wild pets at the same time the ending sequence does not trigger but you are still granted XP, saving you even more time.
The hardest part will be getting your Lil' XT to 25 initially, but that shouldn't take very long at all by working though pet battles normally
Here's a video of the process: