From StrategyWiki, the free strategy guide and walkthrough wiki

The Elite Four are the last bosses. After defeating them, you face the champion. The Elite Four from FireRed and LeafGreen will be covered here.

To beat them your Pokémon should be at least at Level 61 each. The simplest possible team that can win the tournament comprises four Pokémon: One fire, One Water,One Ice and One Electric type. A Water-Ice dual Pokémon will decisively reduce your workload.

• Lorelei:
  • Dewgong (lvl. 54)
  • Cloyster (lvl. 53)
  • Slowbro (lvl. 54)
  • Jynx (lvl. 56)
  • Lapras (lvl. 56)

Lorelei shouldn't give you much trouble as most of her Pokémon are Water types and can be battled comfortably with your electric Pokémon. If you have a Jolteon, definitely bring it along. Use a Fire-type move on Jynx.