The Pokemon after the E4 may seem frightening, but they give a great example of how weak Gen 1-2 Pokemon were. They may be like level 64, but you can one shot them with a level 40-50. Hell, even the trainers aren't much better. The only hard of the post game is the revamped E4 (although Excadrill and Haxorus can sweep all of them), Cynthia, Battle Subway, and the head Pokemon dude. Although with the proper BS items, leveling is actually quite easy.