It's not hard at all, it just demands patience. I never actually went for full stealth run, but there are persons that did full stealth (not noticed ever) and without upgrades. Bosses not withstanding obviously. Now that is hard and challenging.
If you use gun, tazer and fists to disable people, it's pretty much cakewalk. Especially when you upgrade yourself.
If you use gun, tazer and fists to disable people, it's pretty much cakewalk. Especially when you upgrade yourself.