Give the LC Poké Pokérus and the Power Item, then start fishing up Magikarps with the Old Rod in your level bracket. (3-7 for mons hatched on your game. 3-5 for third gen or traded Pokémon) Send the LC mon in first, and now you have to reduce the amount of EXP given by sending in more mons. Depending on if it's hatched, third gen, or traded, you will have to send in more or less mons depending on the karp's level. Once you send in enough mons, kill the karp, and the EXP will be distributed, with the LC mon getting 1. However, it will get 8 EVs from the Power Item + 2 Spe from the Magikarp. Then, you simply have to just keep track of everything as you fish and battle and switch a lot. Use Tamato Berries to erase any Speed you don't want . Also, because there is little room for error, it's always recommended to have at least one extra copy in case of a mess up. By the time I finished, Clysm was at Lv. 3. I then traded her back to SS and used two Rare Candies to get her up to 5.