Here is how you beat the Weak5:
Lorelei:
Dewgong - Thunderbolt, Brick Break
Slowbro - Thunderbolt
Cloyster - Thunderbolt
Jynx - Shadow Ball, Rock Slide
Lapras - Brick Break, Rock Slide
Bruno:
Onix - Surf
Onix - Surf
Hitmonchan - Aerial Ace, Psychic
Hitmonlee - Aerial Ace
Machamp - Aerial Ace, Psychic
Agatha:
Gengar - Shadow Ball
Haunter - Shadow Ball
Golbat - Thunderbolt, Ice Beam, Psychic, Rock Slide
Arbok - Earthquake, Psychic
Gengar - Shadow Ball
Lance:
Gyarados - Thunderbolt
Aerodactyl - Rock Slide, Thunderbolt, Ice Beam, Surf
Dragonair - Ice Beam
Dragonite - Ice Beam
Dragonair - Ice Beam
Gary:
Pidgeot (LOL) - Thunderbolt, Ice Beam, Rock Slide
Rhydon - Surf
Alakazam - Shadow Ball
Gyarados (Blastoise/Venusaur) - Thunderbolt
Exeggutor (Charizard/Venusaur) - Ice Beam/Shadow Ball
Arcanine (Blastoise/Charizard) - Earthquake/Surf
Venusaur (Blastoise) - Aerial Ace
Blastoise (Charizard) - Thunderbolt
Charizard (Venusaur) - Surf/Ice Beam/Thunderbolt/Rock Slide
Just make sure your team has all those moves (STAB is preferred, but these moves on a Poke with a high attack (physical or special) works just as well.