Answer:
option 1(I prefer this one)have the Pokemon hold exp.share then battle really strong Pokemon (like maybe victory road Pokemon)and if it is too weak to fight with them then switch with another Pokemon u will get a lot of exp
or
option 2 u can cheat ....but that may corrupt the game and the internal clock will be damaged u will not get Pokemon in day and night and so many more probs ..........if u r willing to accept and handle thses probs only then go with the second option