For this achievement you need to step and kill 5 of your troops.
The best mission for that is Chapter1 - Mission 2.
You will see 5-6 mines together very early of the mission.
Simply get killed your 2 troops and repeat 2 more times to get the achievement.
Tip: If you haven't finished the campaign, i recommend you do so, because the troops are leveling up on every mission and if you lose them, then you will get new troops with lower rank.
Good luck :)