Walkthrough: After meeting the Battle Legends Red and Blue outside of The Battle Tree, Blue will offer you a battle against them. However, you can only choose to battle one of them. Choose Red, and you will face a very similar team to his back in Pokemon HeartGold & SoulSilver.
Pokemon Trainer Red - Level 74 Pikachu, Level 69 Lapras, Level 69 Snorlax, Level 70 Venusaur, Level 70 Charizard, Level 70 Blastoise
Defeat him to obtain the Mega Stones for Venusaur, Charizard and Blastoise, as well as to earn access to The Battle Tree.