If you want to have tough opponents to beat places to go would be battlegrounds, enemy capital cities of Ogrimmar/Stormwind City not leveling zones where low geared lowbies are leveling usually solo. That would be where the easiest opponenets are found not the toughest.
The tweak that will net you more tough opponents to beat is arenas and the tweak that will let you beat a whole lot of easy opponents is keeping the world pvp flag forcefully switched on. So basically you are asking for a whole lot of easy opponents that you can beat/grief on.