Hi, after playing this game like 3 years ago I decided to give it a go once again some 2 months ago. Now, I'm currently at Kiki and I'm under the philosophy of using a plethora of pokemon; something I hadn't done previously. My starter was Snivy and therefore Serperior is the only pokemon I'm aiming to never replace.
Here is my current team, while climbing Pyrous Mountain:
Serperior - Lvl 41
Grass Pledge
Leech Seed
Mega Drain
Growth
Cofagrigus - Lvl 39
Hex
Will-o-Wisp
Curse
Shadow Ball
Mewostic (F) - Lvl 39
Psychic
Shadow Ball
Charge Beam
Light Screen
Crawdaunt - Lvl 38
Razor Shell
Night Slash
Knock Off
Rock Smash
Donphan - Lvl 42
Magnitude
Rollout
Rock Smash
Slam
Drapion - Lvl 42
Night Slash
Venoshock
Toxic Spikes
Hone Claws
And in the box the pokemon that are around lvl 36 - 42 are: Jynx, Weezing, Camerupt, Mawile, Klang, Ninetales, Piloswine, Noibat, Golem, Granbull, Bearctic, Pangoro
The only notable natures are Adamant on Crawdaunt and Modest on Camerupt