This is for those of you who are stuck at a certain percentage of leaders killed, like I was(93%).
Okay, just got this achievement and these are the steps I completed to get it to unlock after I uninstalled the Armory weapon pack...
1. Went into "settings," then "my profile;" and set it as my "home xbox."
2. Went into "system" and "clear local saves."
3. Went into "power and startup" and uncheck "when xbox is off, storage is off."
4. Went into "network" and "go offline."
5. Loaded up AC: Unity and played through sequence 4 to access the Dead Kings DLC.
6. Played up to memory 2 where I had to free Leon from prison and I got the guillotine gun.
7. Cleared the mob with the gun, then killed the leader and any adds. Waited for it to say "saving" in the top right of the screen.
8. Pushed start and "reload last checkpoint."
9. Rinse and repeat until I got 20 or so kills to be safe.
10. Quit the game and went back online from "network" in the "settings."
11. Restarted AC: Unity and the achievement popped.
This took me almost 5 hours of restarting the game and playing from scratch. Might have been a little faster if I caught the raider leader checkpoint when Leon was in prison, but I didn't. Also, keep in mind, you may have to restart your xbox a few times between steps 1-4. After all is said and done, xbox will ask you which save do you want to use, the one on the xbox or the one in the cloud? If you want all your progress from before, choose the one in the cloud(older).