In GSC, the E4 is not very hard.
Firstly, train all your Pokemon so their level is around 45 or so, if you're
not lazy perhaps up to 50 or so. The highest level in GSC is 50.
Then, make sure you have all the type coverage. FYI,
E1=Psychic = Dark Pokemon. Magneton might be good, there are 2 Xatus IIRC.Thyplosion/Magmar might also help against Jynx.
E2=Poison = Fire. Koga has 3 bug Pokemon. The remaining 2 (muk+Crobat) can be killed using EQ+Thunderbolt/Psychic
E3=Fighting = Psychic/Flying. Onix = Surf
E4=Dark = Only umbreon is pure Dark. Others are mixed. So, EQ = Gengar, Surf = Houndoom, Flamethrower = Vileplume, Thunderbolt = Murkrow
Champion=Dragon. My Dewgong (around 45) killed all Lance's dragonite with one Ice Beam. (the 50 one with two). Gyarados = Thunderbolt, Charizard,Aerodactyl = Rock Throw/Slide