After making your way through Victory Road and reaching Indigo Plateau, you prepare for battle and head north to challenge the Elite Four.
In the first room, you find Lorelei. Talk to her to begin the battle. Elite Four Lorelei has a level 51 Dewgong, a level 51 Jynx, a level 51 Cloyster, a level 52 Lapras, and a level 51 Slowbro. You get 10400 Pokédollars for winning.
In the second room, you meet Bruno. Talk to him to start the battle. Elite Four Bruno has a level 52 Onix, a level 52 Hitmonchan, a level 52 Hitmonlee, a level 53 Machamp, and a level 52 Poliwrath. You get 10600 Pokédollars for winning.
In the third room, you meet Agatha. Talk to her to start the battle. Elite Four Agatha has a level 53 Arbok, a level 53 Gengar, a level 53 Golbat, a level 53 Weezing, and a level 54 Gengar. You get 10800 Pokédollars for winning.
In the fourth room, you meet Lance. Talk to him to start the battle. Elite Four Lance has a level 54 Seadra, a level 54 Aerodactyl, a level 54 Gyarados, a level 54 Charizard, and a level 55 Dragonite. You get 11000 Pokédollars for winning.
After you defeat the Elite Four, you learn that there is one more trainer to battle. Go north to the next room, and approach your rival to start the battle!
Your rival has a level 56 Pidgeot that can Mega Evolve, a level 56 Vileplume, a level 57 Raichu if you are playing Let's Go Eevee or a level 57 Jolteon if you are playing Let's Go Pikachu, a level 56 Rapidash, a level 56 Marowak, and a level 56 Slowbro. You get 13680 Pokédollars for winning.
Congratulations! Be sure to watch the ending. Press a button when it says The End, and new things will be unlocked!