Here is my 100% Achievement Guide for Guard Duty.
19 of the 20 Achievements are story related (Apart from the "Assassins Greed" Achievement)
This is achieved in act 2, after you use the catapult for the first time.
Timestamps:-
Prologue - 3:05
Assassins Greed Achievement - 8:00
Act 1 - 14:30
Act 2 - 33:30
Act 3 - 1:01:40
Hope This Helps!