This is so understandable as to why this is missable, cause if you don't know the area nor the boundaries for the walker "fight". (For me it took many attempts cause I didn't know how far I could go in said area)
In the walker fight, you need to satisfy James, meaning that you DON'T KILL a single walker, only stun and circle around the area so that you don't get killed. After some time you go to push the cart with James, no matter what, you need to STUN the upcoming walker otherwise you'll void the achievement.
If you haven't killed a single walker then James will be grateful and you'll unlock the achievement.