You'll get it playing normally: the assassinations are, as well know in this franchise, when your hero kill some enemy while unseen.
With a stealth strategy you'll unlock it during Sequence 3 or 4, but you can speed it up killing every bad guys you'll meet on your path (the guys glowing in red): since they won't attack you immediately, you have the chance to reach them, kill them hittinh
and continue on your way without any trouble.
Adding HINTS from
DrSchlepenstein (thanks a lot :) )
Mine unstuck while playing the DLC, however not all of the ones that glitched on me did (I had 18 total achievements between the main game and DLC that glitched and the trackers stopped tracking).
You can start the Jack the Ripper DLC and the tracker should continue, but if not, the way I got all of mine to pop was to go offline, delete my console save, and start a new game. Doing so resets the in-game trackers and progress starts accumulating again. Once you've either redone the requirements or done X amount of something for the last % you needed, reconnect to Live and the achievement should pop at the dashboard. It's what worked for me.
So if you're stuck at 94%, you only need like 3-4 more, you just have to do those 3-4 more offline on a new game and when your reconnect to Live he achievement should pop.
You can then delete the new console save and start the game so it downloads the cloud save to your console and you can pick up where you left off. I don't recommend doing this until you've played through the game and gotten everything you can this way you don't have to restart multiple times.
In my case I had the Synchronization points and Completing Memory 6 glitch on me and not pop, so I had to replay 3/4 of the main story missions offline just to get these 2 achievements. Progressing up to this point I completed all the requirements for every other achievement that glitched out on me. So this method should be done at the end so you don't have to restart from the beginning multiple times.