There are many, many strong and unique pokémon here. Things like Misdreavus and Larvitar cant be found anywhere else but here. So heal your team and get your catching arm ready. The path should be simple but if you are having trouble, follow this tip: Keep Heading up. When you get near the end, you will see a trainer that looks almost like the Male Character. It's the character you played as in the Previous games. Speak to him and he will challenge you to a Pokémon Battle:
Red
Pikachu
Lv. 81
Espeon
Lv. 73
Snorlax
Lv. 75
Venusaur
Lv. 77
Charizard
Lv. 77
Blastoise
Lv. 77
After you have finished this battle, congratulations, you have finished the game. But Do Not Stop there. Unless you have a Pokédex of 249-251 Pokémon you havent completely completed it. Go searching for Pokémon and keep raising your team to battle your friends or to battle on Pokémon Stadium 2
