beat the elite 4 ten times then go to unown cave.go to the far corner and take the weakest Pokemon in your party and switch it to the front of your party.(you cant have any level 100 in your party.)
pikachu is not on diamond because pikachu lives in the joto region but i you would care for a pichu i believe you can catch one in the rich dudes garden
you go to this mansion and behind it is a garden look in the grass and you will find both pikachu and pichu pichu is on level 16 and i think pikachu is the same level