This is actually missable.
There are 16 districts to visit as well as associated boss. Only the first playthough that you collect the death card count.
Surprisingly the last district seems the easiest as the commando is located at the very end of the area and you do anything before would not trigger the alarm. He is guarded by a super robot, but just use anything that one shot kill the commando so that he has no chance to raise an alarm
Remember to make a save before enter the district .