Okay, so I still haven't beaten the eighth Gym Leader, but I should be able to do it pretty soon, but I need to know where to train my Pokémon on Platinum. My team consists of;

Empoleon Level 46 - Brine, Metal Claw, Surf, Peck
Jolteon Level 46 - Quick Attack, Thunderbolt, Thunder, Double Kick
Altaria Level 39 - DragonBreath, Take Down, Earthquake, Dragon Dance (in major training)
Rapidash Level 45 - Flame Wheel, Take Down, Agility, Fire Blast
Gallade - Psycho Cut, Magical Leaf (to be replaced w/ Leaf Blade), Swords Dance, Slash

I have an open slot for HM Slaves. I have been training Jolteon in the Route by Sunyshore City against the fishermen, tubers and sailors with water pokémon so that's a good spot I think. If anyone could give me advice I'd be so glad. Thanks in advance.