Are you sure you want the battles to get tougher?

The battles before the elite four are just to make your pokemon level up quickly. Besides, you have to battle a LOT of people in-game and if every second person in that ample amount of challengers was too strong, a non-competitive pokemon player would be rushing to the Pokemon Centre after every battle which would be, quite frankly, very frustrating.

The real challenge always begins after the E4. I thought that was understood by everyone who plays pokemon. .-.

But I do agree that a difficulty setter be introduced to the games, that would please many. (me included)

You can always try out the Sand + Jellicent combo.

Which means, you get a tyranitar and Excadrill(with sand rush) combo, with Jellicent to cover up the entire Water + Ghost type weakness. And since this team is weak to Scarfty, go for a Gliscor too, they have amazing Def.