This is a simple achievement guide which consists of the player saving their sergeant earning him/her 10G. Just follow these steps to achieve the achievement.

1) Load up the mission 'Breaking Point' on either solo or co-op campaign.
2) Play through the mission until you reach the final courtyard.
3) You will see three Japanese soldiers surrendering.
4) Your weapon will be lowered and so you cannot be prepared to shoot them.
5) Suddenly your gun will appear and two of Japanese soldiers will run at Sergeant Roebuck.
6) Quickly shoot both of the Japanese soldiers before they kill Sergeant Roebuck.
7) Unfortunately they will kill Petrosky but that doesn't matter as you've bagged the achievement.

Here is a video tutorial: