Well, first (no offence) you should learn how to use EVs. http://pokemondb.net/ev
Train your Pokemon's EVs; it will help a lot, but isn't required. Then, once your done, train a Pokemon such as the Pokemon above from linkpower22 that are super-effective.
If You have one in your team, great. Obviously because of you "Slowking" tag, you have one high level.
Train it and make sure it has Psychic, for memory TM29 or 28. Surf will do as well, ( it's a HM) STAB moves for both types is important. In responce to the other Elite Four, use Pokemon that beat then too.
Lapras is a killer for Lance (Just watch out for Charizard's Stone Edge), Will can't do much against Spiritomb, Koga struggles against any Ground types (Defence/ Sp.Def recommeded), Earthquake is a killer, especially when Stuntank uses Dig. As for Karen... well, Fighting types kill her, Steel also helps, save for Houndoom.