For the Elite Four, the huge level lead is usually not that important since a few pokemon around level 50 can pull it off, without having to resort to needless grinding.
The Elite Four do require type advantages to win without having to use up too many resources.
Lorelei's Team:
Level 52 Dewgong, Water/Ice, Surf, Ice Beam, Hail, Safeguard
Level 51 Cloyster, Water/Ice, Dive, Spikes, Protect, Hail
Level 52 Slowbro, Water/Psychic, Yawn, Amnesia, Surf, Ice Beam
Level 54 Jynx, Ice/Psychic, Attract, Lovely Kiss, Doubleslap, Ice Punch
Level 54 Lapras, Water/Ice, Surf, Ice Beam, Body Slam, Confuse Ray
As far as type advantages go, Lorelei's team has two major weaknesses:
4x weakness to Electric
3x weakness to Fighting
Lorelei's team attacks using Ice, Water and Normal moves.
Bruno's Team:
Level 51 Onix, Rock/Ground, Iron Tail, Earthquake, Rock Tomb, Roar
Level 54 Onix, Rock/Ground, Iron Tail, Earthquake, Sand Tomb, Double-Edge
Level 53 Hitmonchan, Fighting, Mach Punch, Rock Tomb, Sky Uppercut, Counter
Level 53 Hitmonlee, Fighting, Mega Kick, Foresight, Facade, Brick Break
Level 56 Machamp, Fighting, Cross Chop, Rock Tomb, Bulk Up, Scary Face
I feel like omitting the Onix since they are easily taken care of with Surf.
Major Weaknesses:
Psychic, Flying - 3x
Fighting, Grass, Water, Ground, Steel, Ice - 2x, Water and Grass being a 4x effective move on Onix
Bruno's team attacks with Fighting, Rock and Normal moves.
His Onix attack with Ground, Steel, Rock and Normal moves.
Agatha's Team:
Level 54 Gengar, Ghost/Poison, Shadow Punch, Toxic, Confuse Ray, Double Team
Level 54 Haunter, Ghost/Poison, Hypnosis, Dream Eater, Curse, Mean Look
Level 56 Arbok , Poison, Sludge Bomb, Iron Tail, Bite, Screech
Level 54 Golbat , Flying/Poison, Air Cutter, Poison Fang, Confuse Ray, Bite
Level 58 Gengar , Ghost/Poison, Hypnosis, Nightmare, Sludge Bomb, Shadow Ball
Major Weaknesses:
5x Psychic
3x Ghost, Dark
Agatha's team attacks with Poison, Psychic, Steel, Ghost and Flying moves.
The lead Gengar and Haunter don't directly attack, instead they use stauts inducing moves.
Lance's Team:
Level 56 Gyarados , Flying/Water, Dragon Rage, Twister, Bite, Hyper Beam
Level 58 Aerodactyl, Flying/Rock, Ancientpower, Wing Attack, Scary Face, Hyper Beam
Level 60 Dragonite , Flying/Dragon, Hyper Beam, Safeguard, Outrage, Wing Attack
Level 54 Dragonair , Dragon, Hyper Beam, Outrage, Thunder Wave, Safeguard
Level 54 Dragonair , Dragon, Hyper Beam, Outrage, Dragon Rage, Safeguard
Major weaknesses:
4x Ice
3x Rock
3x Dragon
2x Electric
Lance's team attacks with Normal (Hyper Beam), Dragon, Flying, Rock and Dark type moves.
Rival's Team:
Level 59 Pidgeot, Normal/Flying, Aerial Ace, Sand Attack, Featherdance, Whirlwind
Level 59 Rhydon, Rock/Ground, Earthquake, Rock Tomb, Take Down, Scary Face
Level 57 Alakazam, Psychic, Psychic, Future Sight, Recover, Reflect
Level 61/59/61 Gyarados/Exeggutor/Arcanine
Level 59/61/59 Exeggutor/Arcanine/Gyarados
Level 63 Charizard/Blastoise/Venusaur
The last three pokemon are variable depending on the Rival's starter:
Charizard - Level 61 Gyarados, Level 59 Arcanine
Blastoise - Level 61 Arcanine, Level 59 Exeggutor
Venusaur - Level 61 Arcanine, Level 59 Gyarados
Gyarados, Water/Flying, Hydro Pump, Dragon Rage, Bite, Thrash
Arcanine, Fire , Flamethrower, Extremespeed, Bite, Roar
Exeggutor, Grass/Psychic, Giga Drain, Egg Bomb, Sleep Powder, Light Screen
Charizard, Fire/Flying , Fire Blast, Fire Spin, Aerial Ace, Slash
Blastoise, Water , Surf, Hydro Pump, Rain Dance, Body Slam
Venusaur, Grass/Poison, Synthesis, Sunny Day, Solarbeam, Growth
Major weaknesses:
3x Electric, Rock (Charizard)
Moves used:
Charizard: Flying, Ground, Rock, Normal, Psychic, Water, Dark, Fire
Blastoise: Flying, Ground, Rock, Normal, Psychic, Water, Dark, Fire, Grass
Venusaur: Flying, Ground, Rock, Normal, Psychic, Water, Dark, Fire, Grass
Its confusing to look at the moves used, so I don't think it will have much practical use, at least for the Rival.
Anyway, Boltbeam sweeps the Elite Four, and some of them have glaring weaknesses or sucky movesets (Pure physical attacking Gengar, Hyper Beam, etc.)
I don't have much time to think of Pokemon that might have their tier status changed, but Lapras might deserve a higher ranking do to resisting Lorelei and ability to learn Thunderbolt.