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 Erika, the Celadon City Gym Leader, her team has only changed sightly. Instead on having just her Tangela (Level 33), Weepinbell (Level 33) and Vileplume (Level 34) 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 Vileplume 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, Erika will battle with Tangela, Parasect, Victreebel, Exeggutor and Vileplume this time around.
Just like your first match against her, Erika's Pokemon Team will continue to be very weak to both Flying Type Pokemon and Fire Type Pokemon. Both of these types of Pokemon will cause serious damage to her Pokemon, and they won't take much damage from their Grass Type Attacks as well. Either of these types of Pokemon should do for this battle, preferably at Level 55 or greater in order to match up well.