Alekhine's Gun

Each level can have up to four assassination targets that are part of your objectives (Level 10, ‘Check and Mate,’ doesn’t have any). Both on your compass and when you view your map (), your targets are identified by a number in a circle.

To unlock this achievement, complete one level by just killing your target or targets, no one else. There are multiple ways to do this for each target, so you must be patient and wait for the perfect opportunity as to not alert anyone else (because if you do, you might end up killing them as well).

This can easily be done on Level 1, ‘Ghosts of the Past.’ Refer to “Once Upon a Time” for a video on how to get this and five other achievements in the first level.