At the risk of losing my face, I would really suggest Exp. share; I know you said not to say that, but I tried to train my Swinub without one (and he had good IVs; all stats above 20), but because Swinub is the lowest in its evolution chain, he wasn't strong enough to take stuff on by himself, and the stuff Swinub can take on will give peanuts in terms of experience. I trained up until 37 without evolving him into Piloswine, just so he could get E-Quake earlier, evolved, retaught Ancient Power, then trained w/o Exp. Share and evolved into Mamoswine.
Since you should be around Clair's gym right now, there is actually a really nice place to train on Route 47 (with Swinub holding Exp. Share). Go to that route, and then go into one of the caves, just after the hiker you've fought. There should be some bloke just standing there, go down the ladder, then out of the cave. Surf, turn west, and go up that huge waterfall. Go straight, and turn right. There should be a small patch of grass plus some headbutt-able trees there, and most Pokemon there are level 30+ and give decent experience. The Miltanks are tough, but will give around 1k exp. each.
HGSS is such a pain of a game to level up in. You just need to put in more time than most other games, simply because the level curb is ridiculous.