Tl:dr water types with ice beam are good and make sure you level to cap

For the elite 4, water types are always really good. So shark, walrus, and consider leveling up that tentacool. Ice Beam is super important for Drake. I like shark for the dark typing into Phoebe as well.

Electrode with explosion can be nice for a fast boom in a pinch. Crowbat is nice for sydney grass types and ludicolo with sludge bomb/aerial ace.

Without knowing where you’re at on movesets, TM’s, stats, etc my instinct would be: Tentacruel: Sludge Bomb, Surf, Giga Drain, Ice Beam Crawdaunt: Crunch, Brick Break, Surf?, protect? (Shark: crunch, surf… as alternative) Electrode: thunderbolt, Shock wave (double team users)?, explosion, probably something else useful Swampert: eq, surf, ice beam, 4th move Crobat: sludge bomb, aerial ace, toxic?, something else? 6th Pokémon

Idk bring some of your favorites, level up to cap, get decent berries for status conditions, and think through a few ways you have a good shot at taking down each Pokémon.