Fia has wired a reactor so that it will explode and wipe out Fristad rock (and anyone in it) if she dies. To get this achievement you need to shutdown the reactor prior to killing her. This is a bit easier than it sounds. Once you penetrate into her fortress (there are multiple entrances), the reactor is on the ground floor of the central area (Fia is in a room above it). While there are a number of guards around, you can work your way to the back side of the reactor (more or less under Fia's room) through a variety of paths. Once there you can disable it with your hacking machine.
Once it's disabled, you need to kill Fia to unlock the achievement. It will unlock immediately upon her death, even if you don't manage to escape.