Army of Two

This achievement is pretty easy but if your like me you will not unlock it during the story mode unless you put some kind of focus on the riot shield. If you use it allot and do meleeing with it allot you will have no issues unlocking this.

You will need to pick up the riot shield and walk up to an enemy and press the RIGHT TRIGGER to melee him. If your playing with a AI partner you will need to make him HOLD his position. After 30 kills the achievement is yours.

My favorite spot for both of the shield achievements is on the Aircraft Carrier. As you progress through the level you will reach a check point and will see a riot shield in the corner of the same hallway before going outside on a narrow walkway. There will be several enemies come out at you, once you kill them all, reload the check point (DON'T GO INTO THE DOOR AT THE OTHER END OF THE WALKWAY). It will only take about 2-5 reloads depending how many you have killed in the duration of the game.

Another spot for this achievement is in the first level, Somalia. Once you get through all of the training they take you to Somalia via helicopter. As you make your way through the level you will come to a street with a dead end, which will trigger a cut scene. You come into first contact with Philip Clyde (certified douchebag).

Right after the cut scene command your AI partner to hold position by pushing right on the d-pad (he should stay behind the wall and wont take any of your kills). Grab the car door / riot shield and start moving down the street. Enemies are behind each barrier so be careful. Use the melee attack (right trigger) when close to enemies.

You should get about six or seven kills total. Make sure not to go through the gate on the left or this will trigger yet another cut scene. Instead, pause the game and press restart at checkpoint. This takes you back just after the cut scene with Clyde.

Just follow these same steps, it should take you about 4 or 5 tries. This can also be done for getting 100 kills with sniper, 150 kills with pistol, or 250 kills with assault rifle, since these are the default weapons they give you. It may take a little longer though. I hope this helped.

Video credit goes to https://www.trueachievements.com/lozzy7.htm