Pokémon G/S Walkthrough - Chapter 27

Pallet Town

Professor Oak

Once back in Pallet Town go visit Professor Oak and he'll open up the path to Silver Cave.  Go to Viridian and take the Route toward Victory Road. Now it will let you in where the passages were being blocked by two guys.

From here you can continue out the door to Silver Mountain or go to Indigo Plateau to fight someone that will be a lot easier than the final battle in Silver Cave...

Indigo Plateau - Rival - Final Battle

Anytime after fighting your Rival at Mount Moon you can fight him again at Indigo Plateau right before you go up the stairs to the Elite Four.  After beating your Rival you can go back to that door to the Silver Cave area to fight some pretty tough Pokémon.

Rival
Pokémon	Level	Exp.
Sneasel	45	?
Crobat	48	?
Magneton	45	?
Gengar	46	?
Alakazam	46	?
3rd Evo Starter	50	?
Prize: $5000