I have to say, if you want 0 damage from enemies.. just set the game to easy tbh. :x
I've noticed that i took 0 dmg from enemies as well, but i just thought it was my kickass armor at the time, but reading all this, i realized that they were indeed 5 levels (or more) lower then my level. (and for me, easy is a setting that is never used. I always start games at normal level, otherwise, you can't experience how the game was meant to be.. and it would seem as if it's no challenge)
True, everyone has their own opinion about how it should be played.. and i also have an opinion..
If you use cheats, you're not a Gamer, you're just a lamer wannabe that cannot get through games using your own skill. (there, i said it.)