I know I had to go through the whole game four times. If there is a way to get the beating the game achievements through level select, then it's some bizarre loophole that is not immediately obvious. I never used a guide; I just blundered through it on my own. I just wanted to point out that it's not obvious and requires some kind of trick to pull off.
As for the people with Douglas trouble, try using the spike poles with the skulls scattered around the area. they work exactly like the street signs in the city levels and will impale his head, thus immobilizing him for a while. As far as I can tell, he either won't or can't block it. And don't be afraid to bring some items with you. Also, the mutants drop stuff when they die, so be sure to grab that as well.
I would actually suggest doing a normal run before going into hard mode, just to avoid the harsh learning curve. It's not like the difficulty achievements stack, so you'll still have to do a normal play anyway. No use in going all gung-ho and making yourself more frustrated than you need to.
There's my two cents anyway.