Honestly, I found the best way to get this achievement is to simply get as many of your friends together as possible to run through the game. If you can pull 4 people together, the game will STILL be hard, but really possible. Just keep in mind that running past problem areas will work, so long as you can keep moving and/or not get spotted by the enemy.
A great strategy when using co-op is to have at least one person stick back as far as possible. This way, when someone dies (and they will), there will always be a good spawn point for the dead man to return to the fight. It won't work in all areas, but it's a solid rule of thumb for most of the game.
Focusing your fire on the same targets will also help, especially if living marines are trying their best to stay alive. This helps keep yourselves alive, as well as those marines so they can soak up more damage for you in more important areas of the game.
Either way, if you can persevere (it took my friends and I a loooooong night to get it done), it shouldn't be too hard to finish it.
Just expect to die.