It's simple, just give the pokemon you want to be a level 100 a lucky egg (if you can't find a chansey, exp. hshare will also work). Then when your pokemon is the right level, fight the sunnyshore fishermen. One will give you a total of 11000 exp. Then there is a lady near the enterence to the desert. remath her. If you have a lucky egg, her chansey alone will be worth 600 exp. and in the end you will have 15000+ exp! Plus battle your rival on saturday and sunday. keep at this and your pokemon will reach lvl. 100 quickly and easily