It's none other than Gary, your rival. It seems that he already has defeated the Elite Four and he's standing there, waiting for you! @_@

These are his teams, depending on what Pokémon he chose in the beginning :

If he chose Bulbasaur...

L61 Pidgeot
L59 Alakazam
L61 Rhydon
L61 Gyarados
L63 Arcanine
L65 Venusaur

If he chose Charmander...

L61 Pidgeot
L59 Alakazam
L61 Rhydon
L61 Exeggutor
L63 Gyarados
L65 Charizard

If he chose Squirtle...

L61 Pidgeot
L59 Alakazam
L61 Rhydon
L61 Arcanine
L63 Exeggutor
L65 Blastoise

In my case, his team is the last one shown above(I picked Charmander:). And this is how I did it.

I packed these group of Pokémon to defeat 'im :

For his Pidgeot : Jolteon
For his Alakazam : Parasect(I zapped him with Spore, then Growth him up, then went for the kill with Leech Life. Totally drained him. See, Parasect isn't weak. ^_^)
For his Rhydon : Lapras
For his Arcanine : Aerodactyl
For his Exeggutor : Arcanine
For his Blastoise : Zapdos.

It's good to pick Pokémon that has advantages over his. Some suggestion teams are :

Arcanine
Parasect
Raichu
Blastoise
Ninetales
Electrode/Magneton

*shrugs* Whatever rocks your boat. When I got Mewtwo and took it to the Pokémon League with me(I've beaten them like...a lot of times), I kept irritating Gary's Blastoise by :

Mewtwo used Metronome! Mewtwo used Mega Punch!
Blastoise used Hydro Pump!
Mewtwo used Recover!
Blastoise used Blizzard!
Mewtwo used Recover!
Blastoise used Hydro Pump!
Mewtwo used Recover!
Blastoise used Blizzard!
Mewtwo used Recover!
Blastoise used Withdraw!
Mewtwo used Psychic!

I guess that was a bit annoying but *evil grin* it was worth annoying Gary.

If you win, Prof. Oak will come and explain some things to you and take you to the Hall of Fame(mine's kinda messed up. Bleh). Now you can gloat. ^_^