I struggled a bit but I figured out a good way.
Bring at least 30 hyper potions and at least 10 revives.
Bring a Pokémon that knows the move Thunder Wave.
Bring lots of Dusk balls (because you're in a cave)
If you still have your master ball, then use that.
You can find dusk balls at the shop in Mahogany Town.
Hope I helped.