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HGSS has no "level curve" problem at all. If you fight every trainer you'll be in the 40's when you fight the E4 (which is where you're supposed to be), and if you fight every trainer in Kanto you'll be about 60 (which is where you're supposed to be). The only thing you might conceivably have to grind for is Red, and you get so many advantages against him (EV's, unlimited items, a shitty Pikachu to set up on, Hold Items, pre-knowledge of his team, an easy to abuse AI, even using switch settings) that you can beat him under leveled by ten. And if you STILL can't do it the game gives you the Gym Leader rematches to grind fast.
Oh and look at that, I made a team of good mons consisting solely of Johto mons all available before the third badge.