Get a Darumaka. Grind it to 35. Win Forever.
Some Pokes I found useful for the main game:
Excadrill - It’s a damn metal mole! Plus, it’s really as awesome as everyone says.
Whimsicott - Get one with Prankster and go to town with Priority Status attacks. Madness.
Darmanitan - It completely wrecks stuff. Enough said.
Sigilyph - Very strong Psychic/Flying when you get it. Also has some pretty good abilities. Kinda annoying to fight in-game.
Galvantula - Electric/Bug; Stupidly useful with Compoundeyes. Gives you 90% accurate Thunder! Wrecked stuff in my first playthrough.
Ferrothorn - Wall everything forever. Except maybe Fire, and it’s still kinda good then.
Chandelure - Special Atk. alternative to Darmanitan. Plus the added Ghost-type.
Haxorus - Super high attack and it gets Dragon Dance! Assuming it’s not weak to the opponent, it wrecks stuff. Or you can just Dragon Dance to the max and sweep entire teams. Very much a toothpick.