This is my experience with SAW's achievements.

-First, I checked a console guide for the achievements. They don't coincide exactly, but most of them do. I actually used this "road map" (always look out for them):

http://www.ps3trophies.org/forum/saw/26694-saw-trophy-guide-road-map.html

-There's a HUGE problem with SAW's PC achievements... On console, you can scum-save your way to the achievements: you find an easy checkpoint, you do a specific action, you reload, you do it again as many times as necessary using the same checkpoint, and that's it, you get the achievement. For example:

-Merciless (Stomped 5 minions to death) -> Stomping enemies is quite tricky, actually!
-Professional (Unlocked 20 doors)

But on PC, by (evil) design or by incompetence, every time you get back to a previous checkpoint, when you die or you just reload, you LOSE all your progress.

I repeat: EVERY TIME YOU RELOAD, THE PROGRESS FOR ANY ACHIEVEMENT GOES BACK TO ZERO.

So if you had unlocked 18 doors just by playing in one go and you die, you get back to zero doors. Check those achievements on the global stats page. See how almost nobody in the world has them, even the easy ones? (used 20 Health Syringes, etc) That's because of the glitch.

That is insane, because many achievements are supposed to be reached after a few hours of gameplay. So now you are forced to do everything in one sitting, and you can't die even once.

I tried to use a "trainer" so I didn't take any damage. But you are still insta-killed at some points (if you fail Jigsaw's challenges, if you don't get the right timing to avoid traps...). So... if you really want to get these achievements, prepare to suffer!

GOOD LUCK!