I had no trouble with farming lower levels. However, Progression becomes extremely slow after level 100, which is also when you can prestige. You get 1 crystal for every 10 levels you beat, However, you can only get the crystal for each 10 levels once. Basically, you reach a hard cap in progress, because the enemies scale at an insane rate, but you don't have the crystals to boost gold or DPS.
Either the enemies need to scale much much slower, or we need to be able to farm the crystals repeatedly. It's a great and fun game, but as it is, you can't do any real progression past level 120