what is the best pokemon in pearl to battle if i want to raise sp. atk for piplup with EV training?

EVs never disappear unless you use the EV-reducing berries (they something like "Raises a Pokemon's friendliness, but reduces ___")

As for Sp.Atk EVs, the best place would be the Old Chateau in Eterna Forest. You fight pretty much nothing but Gastly (and maybe Haunter?), so you'll get plenty of Sp.Atk EVs.

Throw in a Power Lens and the Pokerus to make it go even faster.

To raise EVs, simply train against pokemon with high stats in the stat that you wish to be raised. Major sources of high sp. atk. are Old Chateau, which is filled with gastly and haunter, which have very powerful sp. atk. stats. The same rule of thumb applies to all the stats. Also, to raise them faster, you can give them the corresponding vitamins, and have them hold the corresponding power item (power bracer, etc.). Beware of berries that make them friendlier in place of losing stats. It not only lowers the stat itself, but the EV, as well.

It's too late. EV's are gained after every battle. You have already leveled it up meaning you have already filled the EV slots. So it would be pointless right now. If you want to know numel gives 1 ev per pokemon.

http://uk.gamespot.com/ds/rpg/pokemonpearl/show_ms...

This should help you a lot, it is the EV training guide i use, and its the most simple thing to use :)