In Diamond for the first Elite Aaron use primarily Rapidash, Staraptor, and Gastrodon. Rapidash will more then likely KO or 2KO most of the bug Pokemon (depending on the nature).
Staraptor is for any bug types, so your Rapidash doesn't use to many PP or takes much damage if it uses a Fire Type move that misses (Fire Blast) or cause damage onto itself (Flare Blitz).
Gastrodon counters Drapion the best since it resists it's stab moves and can tank the other 3. Therefore I suggest a Ice type move on Gastrodon (i.e. Earthquake).
For Bertha stick with Torterra and Gastrondon and basically spam Grass and Water type moves depending if the Pokemon is water type or not. Grass> Water/Any type she uses.
Water> Everytihng else.
Start With Torterra since her first is Quagsire.
Honeslty your Gastrodon is gonna be your main Pokemon lol. With Flint start out with and use Water/Ground/Ice to wipe out half the team. You can give him/her and break and switch to Luxray for Driftblim but do not use Physical moves or you'll take damage from aftermath. Staraptor can more then likely handle Lopunny with an Arial Ace followed by Close Combat.
Note: Do not use Staraptor on Infernape. It's faster and can KO with ThunderPunch.
Luxray and Torrterra, will be the most useful with Lucian. Luxray and Torrterra both have access to Crunch and therefore can hit hard on most of Lucian's team and 2KO-3KO more then likely. Medicham and Bronzong are the only real trouble.
Luxray can handle Mecicham the best as it get hits neutrally by all it's attacks and can hit with a Electric-STAB move.
Bronzong is gonna be tough to finish off so I suggest Torrterra as it Leech Seed or Synthesis to heal off any damage and keep attacking. If it is to faint but it's taken enough damage (and Lucian has no more Full Restores) finish it off with Staraptor Close Combact.
The same goes for the team in Platinum except for a few differences.
For Aaron:
1.) Take out Yanmega as fast as possible (Staraptor Brave Bird no not Arial Ace)
2.) Try to Save Rapidash for Scizor.
Bertha:
1.) Use Gastrodon on Gliscor
2.) Use Water Type Special Move on Rhyperior as it can sometimes tank EarthQuake and proceed to Avalanche.
Flint:
1.) Stat of with Staraptor to take on Houndoom
2.) Spam Water on his team with Gastrodon. If he begins to use Solarbeam switch out and use Staraptor or something else than can outrun and finish off.
Lucian:
Pretty Much the same concept just replace Medicham with Gallade and be wary of Stone Edge because a Critical will most likely KO. Use Physical Electric type moves on it instead of Special as it can tank those.
That's about it hope I helped 'Guide' you though it.
