Nagato Makes Moving Easy - Won't unlock. :(

Nevermind it unlocked. ^_^

Each time you make a move gets down so if you do a left move come back for nothing you won't get the total at the end it's not easy of cause I'm like you try the shortest way to get each columb.

There is a known glitch that seems to especially effect anyone who did any of the missions through Spartan Ops match making or co-op where it won't unlock.

To unlock it just play any mission again solo, and you can do it on easy too if you wish. Most folks are just playing the very first one over again.

For some it unlocks as soon as they start, others after they finish.

Most of the games I have where you can choose "easy" mode and get an achievement for it. And even if you play on easy mode you can unlock other achievements in the games. I usually play on normal mode unless the game it too hard then I play on easy
mode to get a feel of the game or to help unlock stuff in the skill tree. This way I can play the game on a harder difficulty. For example, I started Wildlands on easy mode and after unlocking some stuff in my skill tree I move to normal difficulty. Now
since I have everything unlocked in my skill tree I m playing on hardcore mode and it aint easy but having everything unlocked in the skill tree was a big help. The same went for games like Wolfenstein, Far Cry, and Metro.

It just has to be a headshot without the enemy noticing you. It's pretty easy, especially in the sewer missions. Just crouch, move forward slowly, headshots! O and make sure you use the regular Steel Tipped Arrows, the others won't work for the achievement.
Hope this helps.