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 it comes to Brock, the Pewter City Gym Leader, his team has changed drastically. Instead on having nothing more than a Geodude (Level 11) and Onix (Level 12) originally, he now has a team of five Pokemon with levels greater than fifty. To be exact, all of his Pokemon this time around will be at Level 56, with the exception of his Golem 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, Brock will battle with Onix, Kabutops, Omastar, Aerodactyl and Golem.
Like before, Brock's Pokemon Team will be extremely weak to Water Type Pokemon and Grass Type Pokemon. Either of these types of Pokemon should do for this battle, preferably at Level 55 or greater in order to match up well. But don't put aside the thought of having a strong Electric Type Pokemon in your Party as well, because, surprisingly, three of Brock's Pokemon (Kabutops, Omastar and Aerodactyl) are actually quite weak to Electric Type Attacks.