first off u need the item lucky egg. Give it to the pokemon u want to train. Now, go to a grass patch with like level 30s, and wait for a swirling patch. If its a pink pokemon called tabunne, if its level 30 and u have the lucky egg it will yield about 2500 exp just for ONE ko!
This is how i grinder to level 40
This is how i grinder to level 40