If you get a party wipe, (so all of your party member dies) your save will be "marked" (but the game won't tell you) and you cannot get this achievement. (even you you replace your "marked" save with an ealier save)
One workaround for automic saves (The game does these when you try to Alt+F4 or close it with Task Manager)
I found is "Sign out" on Windows.
If you ever get into technical trouble (for example a gamebreaking bug), just select Sign out from the Start menu.
(Be careful because this will close all your applications and will sign out of your PC account.)
Sign back in and you can load your last save.