The easiest way to getting this achievement is to go through each of the levels from the beginning to the end. But you do not have to actually win each of them.
What you want to do is start the game up, go into each of the individual levels, and then when you get in a level, you hit the start button, and choose the "Skip Ahead Option."
Keep going into levels, and skipping every single part of them until you get to the level "Downtown."
This is the one level that you do not want to skip. You need to get any medal at all on it, other than Turd Sandwich. It should be noted here that it is much easier to do this with 4 people, as building walls and towers and also attacking enemies is difficult for those not accustomed to Tower Defense games.
*Important*: You can do this all on Casual difficulty.
*Also Important*: If for some reason you do not get the achievement, go to the "Local" menu, and check each of the square boxes in Campaign. each of them should have some medal on it (if you skipped everything, it will be a brown sandwich - turd sandwich - medal). The last mission, "Downtown" should have some medal on it, but the medal cannot be "Turd Sandwich."
For boosting session: To prepare for the boosting session, and to make sure you get your achievement before the session is over, try to play the game ahead of time and skip through all the other levels beforehand. This way, you will get your achievement once your group finishes the last level.
There should be no problems getting this achievement if you follow this guide. I have gotten 48 people (16 groups of 3, with me being the 4th) this achievement. No one had any problems. There was one person in my latest session that did not get it when everyone else did, but he checked his campaign, and he did not play through / skip any of the other levels. He did not message back for help, so I am assuming my advice helped him as well.
From the other solution, in its comments section: "Instantmash also pointed out that skipping all the levels can net you the new Kenny Hoodie avatar award." - Dibs No Joust (26 April 10 at 13:45).
Please write in comments below if there is anything that should be added / fixed in this guide. Grammar / spell checking help is appreciated too.