you can't without a action replay

On diamond if you have an action replay just type in pokerus for a cheat so, do the same with action replay on emerald.

All the codes are at www.classpc.nl

Easy all you need to do is gain experience from beating other pokemon. This is the only way unless you use a rare candy or an action replay or a game shark. Sorry if it takes a while. : )

for Action Replay it is 1-hit kill,max money,quick exp gain,super exp gain(lvls 40+),and you can add more.