max level is 25 sometimes you can fight a pet battle master more then one time, thats a great way to level up your pets, I believe the level range is within five. so if your fighting a level 5 pet, a level 10 and under will give you xp, scaled on how hard the difference is. a good way is to have one 25 pet do the killing, but have your other two that you want to level up, just pop in for one round, you get no xp if you dont. so always make sure you have each pet be in for one round, let your op pet do the actual fighting and kill.
as long as your pet you are fighting with is no more then four levels higher then the pet you are attacking, you will get XP. also each pet has to be summoned at least one round to earn xp.
Still looking for a strat or team on how to beat Major Payne
side note- im 397\400 pets, just three more to go for achievement!