This is a simple achievement as long as you play this game the way it was intended: stealthily.
The prologue does count for this achievement, so what you need is the following:
Stay out of sight and do not leap off from any buildings, and you will be fine. Playing on normal there are no traps in any of the chapters (at least along the main routes) and guards can easily be avoided or, if you like, dispatched without an issue.
Save often, and if for some reason you take damage, reload your save. Be patient and examine the areas thoroughly. I also recommend getting the wrench as soon as possible (vender near of the chapel) as it will open up routes to you and allow you to bypass a lot of guards.
Questions, corrections and neg vote justifications below.