After reaching a higher level with one battle pet, battle a mid level pet that is a few levels higher than the one you want to powerlevel. Drop the bomb from your high level pet (or any ability that does burst damage after a few rounds, I was using XE-321 Boombot), then swap out your low level pet, and watch the bomb destroy the mid level pet. Your low level pet does zero work and gets all the EXP. I listed this in guides and got zero feedback, but after testing this repeatedly it sure does seem like an exploit. I am able to kill a level 10 battle pet with a level 1 and gain almost two levels using this. This is very useful if you want to go for all the leveling achievements.
Try it out and let me know if it helped you. Happy leveling everyone!