Playing the game with an underleveled team

So how about you? Do you guys just level up one Pokemon and go for it or what?

This is actually a pretty large problem of mine that I'm trying extremely hard to prevent in my current playthrough of HG. In nearly every Pokemon game I've played, my starter is 10, or even 20 levels higher than the rest of my team and ends up being the only Pokemon used to beat Gym Leaders. It's the easy way out, even when you're up against leaders that are strong against whatever element your starter represents. I guess I wouldn't know how hard this would be at the Elite 4 since I haven't made it up there in Platinum, but I remember using a level 100 Pikachu to dominate both Yellow and Gold's E4. I've been doing well so far in Heart Gold, with my Croconaw only being 4 levels higher than my next best Pogey. :3