Eighth gym to the Pokémon League
Exit the cave and you will get a phone call. It is Utsugi. Go to him in your hometown and he will give you the Master Ball. You can enter the pokemon league now since you have all eight badges. Leave and surf right/east of your house. You will then see a man and he will give you a map card. This enables you to see the second map which is necessary for your second quest.
Once you have got the second map, go into the cave. You will need the HM07 move where you could surf up waterfalls. Get out of the cave and you are on the blue/red/yellow, so called victory road. It is shorter and there are no boulders or puzzles to solve. Fight all the trainers and you will hit another cave.
This will be the last cave you enter before you get into the pokemon league. It is also your last time fighting your rival. Get out of the cave and you will see a building. This is the pokemon league. Here you will have to defeat the Elite 5. It consists of Itsuki (Psychic pokemon), Koga (poison), Batsu (bruno), karin (agatha's grand-daughter), and Wataru (Lance).
Their pokemon should be level 40 or higher. If you have trained well, it should not be very difficult.