The best place to train would be totem Jungle as the comments above have suggested, my best bet would be to find a Trumbeak and weaken it to S.O.S chain it, this would bring in a fairly reliable number of EXP Points, if fighting Trumbeak would be too difficult, then another good idea would be to fight the Pokémon near Wela Volcano, or even Train Pokémon at Brooklet Hill, your team probably has at least one member that is Super-Effective to one of those types.