1.Get some reviver seeds, Max Elixirs and a bunch of apples.
2.Go to some dungeon that you want to train in.
3.(In the dungeon) Open the menu and go to team, after that, have your team of 3-4 Pokemon to have their Tactics set to "You go the other way" or "Get away from here"
4. Go to a room and Hold B+A till your belly is empty or something is stirring.
5. Go to the next floor if something is stirring and eat something to fill your belly
6.Repeat Steps 4-5 till you want to stop or the dungeon is finished. And thats how you train fast! Good Luck!