This seriously isn't true at all, definitely not in the campaign where you outlevel everyone and even the toughest opponents don't use items except for z crystals, and anyway levels aren't a factor when you battle someone 1v1 like what we're talking about they're all set to level 50. There are no nuclear items, like they're useful but the best ones are defensive IMO and the best offensive ones have serious drawbacks like costing you HP or locking you into that move until you switch out.
When I battle Buddy most of my Pokemon are either level 100 or levels 1-30. Doesn't matter, they're all set to have the stats they'd have at 50. You don't even have to level them up to get most of their best moves anymore, you can just buy heart scales and trade them to tutor your Pokemon moves they learn at higher levels.