RESIDENT EVIL 6
70 Achievements
1,500
40-50h
Civilian Casualties
Defeat the BSAA 100 times.
30
How to unlock the Civilian Casualties achievement in RESIDENT EVIL 6 - Definitive Guide
Some notes, make sure that whoever goes first kills the agent when there is at least 2:30:00 left on the time, if you kill him too quickly, there won't be enough time left for the next person to do it. Also, make sure the NPC is in the middle of the spikes when you activate them, if he is near the edge, he will fly out of them instead of dying instantly.
Defeating the BSAA translates to killing the BSAA Rookie Agent. This is best done on Steel Beast as the J'avo run and kill faster than the zombies on other maps.
Since a standard match of Siege Mode has two rounds, both teams can work on this during the same match. Have your Human partner(s) choose Helena, since she has grenades in her loadout. While playing as the Human team, throw all three of your grenades at the Rookie to lower his health. The player on the Creature team should now rush towards the Rookie and finish him off. This will count as one defeat over the BSAA.
When the teams switch sides in Round 2, now the other player can kill the Rookie in the same fashion. This method will net both you and your boosting partner about 20 'Defeats' per hour.
You can check your progress by opening the Records in the Main Menu and going to 'Extra Content -> Siege -> BSAA Agents Defeated.'