U can also get a item called " exp share "
When I have a pkn I'm trying to lvl up fast I put them in my team of lvl 100s and then the lower lvl pkn will receive all of the exp points that the lvl 100 would normaly get. ( because Pokemon cant lvl up higher than 100 all of the exp points would go to the Pokemon holding the exp share.
But if you dont have lvl 100s then your Pokemon holding the exp share will only get half of what is given.
I hope my answer has helped you a lil ( or alot )
And good luck with ur game :)