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 Blue, and you will face a very similar team to his back in Pokemon HeartGold & SoulSilver.
Pokemon Trainer Blue - Level 69 Alakazam, Level 69 Machamp, Level 69 Aerodactyl, Level 69 Exeggutor, Level 69 Arcanine, Level 69 Gyarados
Defeat him to obtain the Mega Stones for Venusaur, Charizard and Blastoise, as well as to earn access to The Battle Tree.