Gears of War 3

Go into Beast, select 'Private', select any other settings you like [They don't matter], and then invite 4 other people, you can also use a guest for this, so 3 people with 2 using guests is enough. [These people can already be in your party, they don't need to be invited at this point if they are there already ^^]

Once you're at the map select screen, choose one of the 3 'Horde Command Pack' maps. [Blood Drive, Rustlung and Azura].

Then start the game and complete a wave! It should then pop.

Best thing to do is get 5 of you together and rotate who the Host is, you can all get this in a matter of minuets.

If you have 2 people signed into the same machine they both get the achievement from one game if one of them is the host.

If you want to get two birds with one stone here, you can all mooch on over to 'Private' Horde and do the same again for 'The Host with the Most'



Edit: Thanks to Iam918 for the info on needing to finish the first wave.
Edit: Thanks to liner bronson for the info on using guests.
Edit: Thanks to ExpletoryGibbon for the info on two people on the same Consol.