OK so im not 100% sure if this is a bug. But you can have 2 25 pets and 1 level 1 pet and kill pet battle trainers all over like any of them. Hit them once with the level 1 pet then pull out a 25 pet and kill them off and you get like 2+ levels per which is pretty crazy. I don't have the hat from the 40+ trainers yet so you could even get more exp. I have got too like level 10 in 5mins flying around too the trainers. Thanks and I hope you enjoy!