@froppiccino Furthermore, the exp share is still attached to one pokémon only in that game. The old way to level up is to send the pokémon you want to level up first and switch on first turn to a better pokémon.
In that game, the larger the difference in levels when defeating a pokémon, the more exp points you get (if your pokémon is weaker than the foe, but you get less exp if your pokémon is stronger). So try to level up in areas where foe pokémon are stronger than yours.
Another tip is to find and defeat Audino, as they give the most exp points. They appear on any road with grass, in rustling grass.
In the post game, the white hollow tree and the black tower are also good places for leveling up, but are rather challenging.