All you have to do is grab a friend and play through the game with them, from beginning to the end. You don't have to do it all at once, so don't kill yourself over it. You can split it up by chapters if you really need to. Any difficulty, Standard or Arcade.
If you have xbox live and do not have a friend locally, then you can go to arcade and make/join a public match on whatever Act or Chapter you need done. This only works if 1 or more other people are playing with you.
If you, for some reason, don't have a friend and you do not have xbox live gold membership, but you have a second controller, then you can plug that 2nd controller in and set up that profile for Casual (or any other difficulty if you want). I said Casual, so that the dummy account has less of a chance of dying. With this method, you will have to operate both controllers occasionally, but allow you to get this achievement by yourself.
I am playing the campaign through on Standard Hardcore Co-op (locally), but it's really your preference on this one.
Enjoy the game Gears!