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 Red, and you will face a very similar team to his back in Pokemon HeartGold & SoulSilver.
Pokemon Trainer Red - Level 70 Pikachu w/Light Ball, Level 65 Lapras, Level 65 Snorlax, Level 66 Venusaur, Level 66 Charizard, Level 66 Blastoise
Defeat him, and you will gain access to The Battle Tree.