Army of Two

To get this achievement you must break 300 objects in either DLC mission 7 or 8, as the achievement states.

While you can break lots of items in Mission 7 the best place to break items is in Mission 8. Even doing this in mission 8 it will take 2 times and if your playing with a co-op partner it will take 3-4 times since the achievement is for each player and you will need to break a total of 600 items in co-op.

Make sure to shot EVERYTHING to include chairs, clocks, electrical boxes, laptops, TV screens, fire extinguishers, cameras, etc. Make sure you go in ALL the side rooms and offices.

In one of the rooms, that you can't miss, there is a room with looks like a large picture of you and your partner's face but if you get closer they are really 40+ screens that you can shoot. Also in that room there are plenty of TVs and other items to shoot.

Just shoot everything and then reload the level and start again. If it looks like you can break it, shoot it.

This achievement requires you to break 300 objects in either mission 7 or 8 (of the DLC).

Do NOT reload checkpoints over and over*, the game does not save your progress until the next checkpoint. However, to get this in one run there is still a convenient way. Do level 8, rush to the computer room. When you are on the 2nd level shoot out all the monitors, windows, CCTV Tapes, glass, etc. Go on til it says checkpoint. Once it says checkpoint Pause and hit Restart From Checkpoint. All of your progress saved since you hit a checkpoint, but the restart respawns all of the stuff. Break everything again and proceed to the next check point. I believe you get 3 opportunities to break everything in the computer room using this method.

If you run out of ammo just quit out and before you hit continue from checkpoint go in to weapons and pick 2 new ones. This will cause the game to give you fresh ammo.

Also, once you're past the computer room and in to the vault, before you run in to the middle go around the edge and destroy all of those CCTV tapes as well (if you need them)

Using this method you can get 300 broken items in one run without having to restart the level.

Remember the key is you MUST reach a checkpoint for it to save progress. I even tried dying on purpose to reload the previous checkpoint and could never get the achievement to pop.

* Some claim reloading works, however the majority of people find this method DOES NOT WORK. There is no point reloading when it is easy enough to just do it my way and not risk having a problem.