You get so few points for leveling. Even if you add 5 levels plus skill points to 70 heroes, you'll only get 49k, which is a small fraction of the top progress reward. Most points come from spending, which you can do easily by buying silver chests, using shop refreshes, or crafting items.
I'm not sure why you'd level ALL your heroes. If you're spending enough real money on the game to keep them promoted as well as leveled, then you must have tons of gold to get contest points. If you haven't been spending all that money, then you gain nothing from all those weak but high-level heroes. Though if it's fun for you, why not? It is a game after all.
Finally, the prizes for this contest aren't very good. Once you've reached the top progress reward (which is easy), the rank rewards aren't worth fighting for. Then again, if you spend all your gold leveling heroes that you never use, maybe you will have trouble with this contest.