As someone who has made it to the final boss, the worst is behind him, in my opinion. World 4 was the difficulty spike peak for me. Once you get your head around the enemy and level design in World 5, it shouldn't be too difficult, but then again I didn't have this large a break from the game, which will definitely be a factor when the next Breaking Brad comes around. As for World 6, it's just all the enemies and obstacles from Worlds 1-5 thrown in there together as a sort of summarization of what you've learned playing the game. Also the rooms are really quickfire and there's a checkpoint for each one.