Pokémon Brilliant Diamond and Shining Pearl have some post-game activities for players to perform after defeating the Elite Four. One of them is to simply face the Elite Four again, only this time they are much more powerful. If you are looking for perhaps the most difficult challenge that Generation IV remakes have to offer, this guide will show you how to reassemble the Elite Four.
To reassemble the Elite Four, simply head to the Pokémon League building where you first fought the Elite Four and throw the gauntlet once more. Each Pokémon in the Elite Four squads will be a higher level this time around, with levels ranging from 65 to 78. All of their Pokémon experience a fairly sharp increase in power, so first make sure your Pokémon are of a fairly high level to pursue a revenge.
If you pass the rematch, you are almost ready for the final challenge of Shining Diamond and Shining Pearl: the second rematch with the Elite Four. To go through this grueling test, you'll first need to unlock the National Pokédex, after which you should be able to take on Heatran at Stark Mountain. From here, capture Heatran, return to the Pokémon League building, and go fight the Elite Four again.
This time, each member of the Elite Four adds a new Pokémon to their party, so that all five members have a well-stocked group of six Pokémon. Each Pokémon's level is also drastically higher than before, with levels ranging from 75 to 88. You'll need to make sure you're at least level 90 or higher if you want to beat the Elite Four without putting too much effort into it. The consensus among the Pokémon community seems to be that Cynthia's second rematch is among the hardest, if not the hardest, to be found in any Pokémon game, so if you're looking to put your battle skills to the test, it's here you will want to go.