If you prefer video solution, please find below.
This is in the first mission:
Osaka Castle, shortly after you are introduced to the old sniper guy - Takuma, who is also a playable character from this point.
By using the ability of Takuma, wait until the guy who is running back and forth, joins to the two others below the hanging planks and shoot the crane's suspension. Easy unlock!
Warning: There are 3 other achievements unlockable at this same exact point with minor effort, check these guides as well: