
Skillwise go with something like this:
http://underrail.info.tm/?build=AwGlEYBYQJgVhAZhOZ5TIJxJA1DFQJE8SUzNwQB2akSIsETZa6CCeJxnoAWeapons of choice - Shock sarrated tungsten steel knife and .44 pistol with rapid reloader (electroshock pistol would be better but it doesn't suit Sam Fisher). Your armor should be damage threshold focused tactical vest with 0 armor penalty plus tabi boots.
You also might want to play with oddity xp system as it allows you to sneak around without killing and still not fall behind xp wise. In that case you could focus more on stealth skill and swap Hit and Run with Interloper feat.
Basic strategy: When possible open up every fight with Cut Throat, if enemy is still alive and very though use execute on him, otherwise save it for your next enemy that gets incapacitated with Cheap Shots. On your second turn you'll have plenty of MP for Blitz. Eviscerate is a good finisher for tough enemies when you deal little damage but manage to stack many bleeding wounds. Applying Expose Weakness before that helps too. Make sure you eat an eel sandwich before combat so you have a total of 14 dexterity. You need those to maximize your attacks per round - 12 attacks for the knife (including Taste for Blood bonus) and 4 for the .44 pistol.
If something is unclear with the chosen feats just ask but look here first:
http://www.underrail.com/wiki/index.php?title=FeatsI also made a custom Sam Fisher portrait you can freely use:

Just drop both files from the attachement below into C:\Users\[Name]\Documents\My Games\Underrail\CustomPortraits folder.