Details: After meeting Red and Blue outside of The Battle Tree, Blue offers 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 65 Alakazam, Level 65 Machamp, Level 65 Aerodactyl, Level 65 Exeggutor, Level 65 Arcanine, Level 65 Gyarados
Defeat him, and you will gain access to The Battle Tree.