Last night, I was feeling extra petty. I decided that on the next day, I would open the Nintendo DS I gave my sister and beat Pokemon Platinum for her after she had given up on the game for a good 2+ years, right before the Pokemon League.
And so I did, next to her, with her team that was underleveled, despite training I did for her in preparation for the league. I then preceded to dance to the credits, at some point dabbing, to confirm that I'm an idiot.
And that my friends, is proof you don't need level 60s and 100 revives to beat the Sinnoh league. Don't listen to the recommended levels and amounts of items you see on like WikiHow and Bulbapedia, they're exaggerating how tough these battles actually are. It's gen 4, not Dark Rising.