Getting back on-topic.
Sydney - Blaziken would play the best offensive game here, but Crawdaunt/Sharpedo would be a formidable opponent with Surf, so grab a Grass or an Electric-type to back it up. Lanturn would be great there. Ensure it has Thunderbolt [you can get it from Wattson from saving New Mauville or buy it for 4000 coins at the Game Corner] For everyone else, Blaziken would be the champ.
Phoebe - Ghosts. Sableye. Challenge. A Dark-type would be good here. Sharpedo, or any Pokémon with Crunch would eliminate the Dusclops and Banette, but for Sableye, use Aggron. It's Steel-type traits would help you, and Iron Tail can eliminate Sableye in one blow.
Glacia - Ice-types are weak to Fire, Fighting, and Rock. Blaziken has two of those traits, and can learn Rock Slide as a baby. Use Sky Uppercuts, Blaze Kicks/Flamethrowers to eliminate the Glalie, and Lanturn's dual-type capabilities provide great resistance and attack power. Thunderbolt the Walrein and Sealeo and you have a match!
Drake - Does your Milotic have Ice Beam? Ensure that it does. If not, grab a Dragon-type of your own, or an Ice-type with powerful Ice-type moves. Dragons hate Dragon attacks and Ice attacks, and are resistant to numerous other types, such as Fire, Electric, and Water, so Blaziken and Lanturn are your last resorts here. Aggron could provide some defense, but not much here. Milotic is great, if it has Ice Beam.
Wallace [Emerald-only] - Heal up your Lanturn with some Max Potions or something, because it's going to be used a lot here. Grab a Grass-type for Whiscash and a Flying-type for Ludicolo, and use items if necessary. It'll be a tough match, but you can get through.
Steven [Level 50-58 RS 70 - 78 Em] - Magneton, if you don't have one, will be good for Skarmory. If not, Lanturn is okay. For most of Steven's team, it is weak to Fighting and Fire. Blaziken's Sky Uppercuts and Blaze Kicks/Flamethrowers will do the job here. If not, Lanturn's Surfs will do wonders as well. Ensure you have a ton of Revives and Potions, because in this battle, it's tough, especially in Emerald, when his Pokémon are 20 levels higher than that of RS.
+Chaos Blade+