I agree with the above statement too.
The first Rocket grunt uses a team of 6 Pokemon with Explosion
Pineco, Electrode, Geodude, Weezing, Koffing and Graveller. A Ghost type will be good for this. Try Haunter and when geodude or Graveller come out you can use Skarmory or something.
The second rocket grunt uses The (Toxic/Whirlpool/Clamp)/Protect/Double team/(Fly/Dig/Sand-attack) combo.
In Round one they only use Toxic. Use a Poison type (maybe Haunter again?) and a Steel type with Eathquake (for when they use dig). If you poison the opponent too, you will both faint on the same turn too.
the third trainer was an executive. The whole team has Frustrationa dn the whole team also has thief. Try using a steel type Pokemon again (with a useless or nor hold item). Skarmory or Forretress will also be useful.
The last trainer is another executive. I can remember him having a Victreebel, a Houndoom and a Wobbuffet. He has a Tyranitar (although that may be in round two). Wobbuffet is not too hard in GSC. Victreebel and Houndoom are Sunnybeamers. Raindance will be useful. Try a Chinchou. A Haunter will be useful for Wobbuffet.