This is my team i only used about half of them and still somehow managed to deafeat the Elite 4 (using revives and max potions too of course
Magneton lvl 54
Thunder
Tri Attack
Supersonic
Spark
Pidgeot lvl 43
Wing Attack
Fly
Featherdance
Quick Attack
Hitmonchan lvl 42
Sky Uppercut
Mega Punch
Ice Punch
Mach Punch
Charizard lvl 59
Flamethrower
Slash
Wing Attack
Blast Burn ( i got blast burn after i beat them, i used metal claw before so BB will help you out even more)
Starmie lvl 56
Cosmic Power
Pyschic
Surf
Recover
Weepinbell lvl 56
Poisonpowder
Slam
Acid
Razorleaf
(better choice to just evolve him sooner than 56 i am still yet to do that)
Basically i use Magneton to kill Lorilei's Dewgong, Cloyster, Slowbro, and if i can Lapras, Use Charizard to take out Jynx easily.
If Magneton fails to kill Lapras or if hes about to die, make sure u use Supersonic to give an edge to Hitmonchan, Hitmon chan works well against Lapras becuase of its High Attack and Special Defense, which completely shut out Lapras.
On Bruno Use Starmie or Weepinbell to kill the Onixs
Using Starmies Psychic isnt all that useful against Hitmonlee or Hitmonchan because of their Hi Sp. Def, so i use Charizard or Pidgeot against them and use Starmie for Machamp.
Against Agatha use Charizard to kill the two Gengars, and Starmie's Psychic on the rest.
Against Lance, this is very difficult because u have no ice or dragon moves, use magneton to easily kill Gyarados then use Hitmonchans Mega Punch or Ice Punch to deal more damage (ice punch wont help much because of Hitmonchans low Sp. Attck)
Mainly use Charizards wing attack and slash to kill the two Dragonairs quickly. When Dragonite comes out, use Supersonic to confuse it then try to paralyze,burn, or freeze it with tri attack and finish it with some more wing attack/slash.
When fighting Gary....
Magneton is first to easily take out Pidgeot
Starmie for Arcanine
Charizard for Alakazam, and Exeggutor
Magneton and if u need to Weepinbell for Blastoise
Charizard for Magneton, (i cant remmeber if he has one, but just use the weakness to take him out)
BE SURE TO STOCK UP ON 20 OR MORE REVIVES AND AT LEAST 20 MAX POTIONS/FULL RESTORES AND 20 FULL HEALS ALSO AND THEN YOU SHOULD BE ABLE TO DEFEAT GARY, IF U DIDNT START OUT WITH CHARMANDER....TOUGH LUCK MAKE A NEW GAME