The achievement requires you to create a custome difficulty using a list of modifiers that make the game increasingly difficult. Heres the list that I used to give me the best chance, as well as tackle the "A Moral Victory" achievement and "Clear Headed"
Chapter Saves Only - OFF (save EVERYWHERE to avoid annoying restarts or mistakes)
No Focus - ON
Stealth Takedowns Only - ON
No Reticle - OFF (For the sake of 30pts just makes it easier to set up ropes and water arrows)
Speciality Arrows Only - ON (No lethal arrows needed for this run)
No Food or Poppies - ON (No damage should be taken or focus used)
Zero Damage - ON (Again, same as before, no damage should be taken anyway)
Expensive Rescources - OFF (Restock arrows for cheaper is a must)
Slowed Movement - OFF (Just no)
Iron Man - OFF (We dont want to be restarting the game now do we)
No Upgrades - ON (You can still buy the wrench for 1000g and the wire cutter for 450g which is all you really need to stay stealthy)
No Kills or Knockouts - ON (Helps with the achievements, and we dont need to be seen anyway)
No Alerts - ON (Same as above, its not needed to be seen, just be wary of birds and dogs and be mindful of where the guards are)
This should give you a rating of 725 which will net you the chievo at the end of the chapters. The most important thing to remember is to save often, the guards can seem to have varying levels of wariness, so if youre unsure of a route, save, try it and if you fail, reload. The game isnt difficult to beat with these parameters, patience is just key. Try to amass as much money in chapter 1 so you can buy the wrench and wire cutters asap, these will help you navigate maps a little easier. Chapter 4 provides an area of difficulty, when going to save basso the keep will be on fire, the smoke does damage to you if you stay in the area too long, you will have to sprint to the wheel and open the gate as fast as possible, it can be done, just dont hang around
EDIT: For the thief taker general, you have to be quick and mindful of your moved. First off, dodge and swoop left to the crank, turn it as fast as you can to open the first set of bars covering the door. Watch as he is firing fire arrows around, making sure to stay out of his view continue left around to the other side. There will be another crank that you need to open. just be mindful of his actions, he tends to just stay in the centre, but his firing can be random. Once both cranks are open, from where you are, fire a water arrow at the torch above the exit door and swoop over. It is a five tumbler lock, so be patient because if you miss one, the general will hear you and give you a nice fire arrow in the back. Once opened, get out of there! It can be frustrating but it can be done, you just need to be efficient and dont panic.