In the last chapter there is a challenge where you need to defeat Count-Duke with your secret attack (LB+X). If somehow you manage to do this but he hits a wall or falls down stairs it will not count towards the achievement and you'll most likely have to restart the whole mission because the load checkpoint will not spawn you prior to the fight.

To go around this, you can do the following steps to backup your save and just reload it if you somehow fail the challenge.
1 - Reach the room where you fight Count-Duke
2 - Go to %LOCALAPPDATA% \EnGarde\Saved\SaveGames
3 - Backup all the files inside
4 - Fight Count-Duke
5 - If you somehow kill him but the challenge is not completed, Close the game
6 - Copy the backed up files and replace the ones in the directory mentioned in step 2..
7 - Start up the game and press Continue, you can now fight the boss again.