Thought i'd add a solution seeing as there isn't one.
Sounds hard but is easier than the similar achievements in AC China and India.
No alert simply means you can't get the message 'Alert' which appears in red if you alert a guard by being in their line of sight or by them finding bodies. On Plus Hard you only see their vision cones by using eagle vision.
As opposed to the AC India one you can be seen just as long as you don't get the alert message, so it is not necessary to get 'Gold' throughout, some silvers are fine and you can check after each section as it tells you if you have alerted anyone.
Pointers: If you alert guards then pause and restart the section.
- You can kill guards as long as the body isn't found, so in areas with multiple enemies you can use cover, hide bodies or use helix kill.
- Smoke grenades are your friends - whilst not strictly necessary it's advisable to get enough shadow golds in the 2 levels that increase smoke grenade capacity in order to get the upgrades where you can carry 6 smoke grenades. This makes life very easy. At least get to 4 smoke grenades, that's enough, as I think the other upgrade is level 8/9 so a bit late for it to make that much of a difference.
- Running slide kills are useful also when timing is tight/the guards with the circular vision cone.
- As I said above you can be seen so that a guard investigates as long as you dont get an alert...it is useful and necessary to draw guards away like this a few times in the game.
There are some exceptions where you cannot help but get alerts however - I could not for the life of me work out how to get through level 7 without alerts.
- Straight after the second sniping section, as the girl, you run into a building, open a window, and the 2 guards get shot - it gives you an alert. I tried all sorts, whistling etc and none worked. You are allowed that alert right there.
- A little further on and you climb up to where there are 3 guards, a locked door, and you open the window for 2 of the said guards to be shot. Again you are allowed that alert.
For all the sniping parts it is possible to get no alerts, either by finding a 'route' and shooting guards in a certain order so no bodies are found, or by shooting near a pair of guards to draw their attention and then taking the pair out quickly so that it gives no alerts.
You can check your progress - there are 10 levels so one level is worth 10%.
Vid if you are stuck.