Elite Four & Gary

#1 Lorelei
Monsters:
Dewgong Lvl. 54
Cloyster Lvl. 53
Slowbro Lvl. 54
Jynx Lvl. 56
Lapras Lvl. 56
Best Pokemon to Use: Electric or Grass

#2 Bruno
Monsters:
Onix Lvl. 53
Hitmonchan Lvl. 55
Hitmonlee Lvl. 55
Onix Lvl. 56
Machamp Lvl. 58
Best Pokemon to Use: Water, Flying, or Psychic

#3 Agatha
Monsters:
Gengar Lvl. 56
Golbat Lvl. 56
Haunter Lvl. 55
Arbok Lvl. 58
Gengar Lvl. 60
Best Pokemon to Use: ???Psychic???

#4 Lance
Monsters:
Gyarados Lvl. 58
Dragonair Lvl. 56
Dragonair (again) Lvl. 56
Aerodactyl Lvl. 60
Dragonite Lvl. 62
Best Pokemon to Use: Ice or Dragon

Gary

Your rival has already beaten the four and now you must beat him. His monsters will change depending on which monster he took in the beginning of the game from Prof Oak.

First half of the team:
Pidgeot Lvl. 61
Alakazam Lvl. 59
Rhydon Lvl 61

Second half (chose Squirtle)
Gyarados Lvl. 61
Arcanine Lvl. 63
Venusaur Lvl. 65

Second half (chose Bulbasaur)
Exeggutor Lvl. 61
Gyarados Lvl. 63
Charizard Lvl. 65

Second half (chose Charmander)
Arcanine Lvl. 61
Exeggutor Lvl. 63
Blastoise Lvl. 65

(You may beat the Elite Four as many times as you'd like to gain levels and get some more money.)