Not necessarily a gym leader, but Sirius at Tanzan Cove. (Delete if not allowed)
Not sure why I'm having such a hard time beating him. Once I get to his golisopod, the whole battle goes downhill.
Luxray (54)
Charge
Crunch
Discharge
Thunder Fang
Typhlosion (51)
Rock Smash
Flamethrower
Smokescreen
Lava Plume
Camerupt (50)
Lava Plume
Earth Power
Yawn
Earthquake
Meowstic (female, 51)
Light Screen
Signal Beam
Future Sight
Psychic
Toxicroak (50)
Poison Jab
Mud Bomb
Sucker Punch
Drain Punch
Mawile (52)
Iron Defense
Iron Head
Play Rough
Crunch
I also have Dodrio (50), Sharpedo (47), Donphan (45), and Swoobat (45) in my PC