Fire, flying, ice, and there could be some more.
Fighting/Bug
Bug, Ground, Psychic, and steel
First see what Pokemon they have then remember the Pokemon and the type they are. Get the types that's good against the Pokemon they have and train them to level 100. Water beats Fire Water beats Rock Fire beats Grass Fire beats Ice Fire beats some Bug Grass beats Water Grass beats Rock Ground beats Fire Ground beats some Bug Psychic beats ghost ... ... ...
In Pokemon certain types can beat others and here are my examples: Water beats Fire, Fire beats Grass, Grass beats Water, Dark beats Ghost, Water beats Electric, Water beats Ground, Water beats Rock, Grass beats Rock, Grass beats Ground, Fighting beats Grass, Flying beats Grass, Physic can beat Grass, Ground beats Fire, Fire beats Steel, Fighting beats Dark, Bug beats Dark
Fire beats grass bug steel and ice Grass beats water rock and ground Ground beats electric fire and rock Rock beats flying dragon and steel Water beats rock fire and ground Flying beats grass bug and fighting Bug beats psychic and grass Electric beats flying and water Dark beats dark psychic and ghost Psychic beats fighting Ghost beats psychic Dragon beats dragon Ice beats dragon flying and grass
Grass-type Pokemon are weak against Fire, Flying, Poison, Ice, and Bug-type attacks.
Fighting/Bug. A.K.A. Heracross.Fighting and bug. :)
Bug, Ghost, and Dark type are super-effective (2x) against Psychic type Pokemon (assuming it is pure Psychic, such as a Mew)
fire beats bug fighting beats ground rock or water beats fire fire beats phsicic and then there is a mix-upfor the champion
Normal doesn't beat anything Fire beats Grass, Ice, Bug and Steel. Water beats Fire, Ground and Rock Grass beats Water, Ground, and Rock. Electric beats Water and Flying. Ice beats Ground, Flying, Dragon and grass Fighting beats Normal and dark Poison beats grass Ground beats Fire, Electric and Rock. Flying beats Grass, Fighting and Bug. Psychic beats Fighting. Bug beats Grass, dark and psychic Rock beats Fire, ice and Flying. Ghost beats ghost and psychic Dragon beats Dragon. Dark beats Ghost and Psychic. Steel beats Rock. Hope this helps!
You can't other than a starter chimchar. But in Pearl you can find a murkrow at night (flying beats grass) or find a burmy on a honey tree and evolve into mothim (bug-flying beats grass) if it is a male. if it is a female it will be wormadam (bug beats grass).
Bug, Ghost, and Dark type are super-effective (2x) against Psychic type Pokemon (assuming it is pure Psychic, such as a Mew)