After defeating the Elite Four and becoming the newest Kanto Region Pokemon Champion, you can battle all of the Gym Leaders in a rematch on a daily basis back in their given Pokemon Gyms. Each Gym Leader will have a team far different than the one you faced originally, usually being much stronger than before with additional Pokemon too. On top of this, you will earn Experience and Money every day you defeat them.
When is comes to Misty, the Cerulean City Gym Leader, her team has changed dramatically. Instead on having nothing more than a Psyduck (Level 18) and Starmie (Level 19) originally, she now has a team of five Pokemon with levels greater than fifty. To be exact, all of her Pokemon this time around will be at Level 56, with the exception of her Gyarados that will be at Level 57. If you put this into perspective, you will quickly realize these are similar numbers to the Pokemon your Rival had during your Championship Match! In all, Misty will battle with Golduck, Dewgong, Starmie, Vaporeon and Gyarados.
Just like your first match with her, Misty's Pokemon Team will still be extremely weak to Electric Type Pokemon and Grass Type Pokemon. Either of these types of Pokemon should do for this battle - Electric Type Pokemon being the better of the two since her Dewgong has a few Ice Type Attacks that Grass Types hate - preferably at Level 55 or greater in order to match up well. It shouldn't take much more than this to defeat Misty in your rematch against her.