If you have beaten the Elite Four, go back and defeat them again with the Pokemon you want to level up holding an Exp Share.
Get the Vs Seeker and go west of Lavender Town until you get to the line of people. Use the Vs Seeker and when you're done, recharge it, go back and do it again. Repeat this until your Pokemon is on your desired level.
also if you have beat the elite four and can get to the seven islands go to seven island and go down their path tell you come to 2 trainers on she be a girl with a chansey the chancey knows theses moves hardboiledegg, minamize, defencec curl and egg bomb
battling her and winning should give you about 3000 exp and the other guy before her has 2 exagutor and one tangla this guy is very easy but that's only if you have a high lvl fire type if you are going to train any thing else their remember to have lots of parzely awaken and antidot it should help (he like making you PZL so buy more of them ) that should help you get to lvl 100 fast i used it and it worked for all my lvl 100's. good luck (if questions ask me at email@example.com)