Load up the
4th mission - "
Siege of St. Cross" -- set it to Casual if you want to make this easy on yourself.
This achievement can be unlocked in 5 minutes or less. Easy.
At the start of the mission you will be directed to the back of the church. The guy on the mounted machine gun (HMG) will be taken out as you enter the area. Do not get on this (with the X button) at any point. Just stay away from it. Take out the enemies and stay alive, eventually they will fall back and you will get the achievement immediately.
You can also get the
Red Barrels achievement pretty easily here:
The Red Barrels achievement in Enemy Front worth 18 pointsKill 20 enemies using explosive environment objects (Singleplayer)