Forum:Nothing Special Discription

I have a quick question. The description on this site for the 'Nothing Special' achievement says that if you play through he Dead Air campaign with a friend, that you can take damage and then when you get to the finale simply call a vote to return to the lobby, start the campaign on the finale of Dead Air and as long as no special infected harm you, you will get the achievement. Now, does it have to be over xbox live? or can it be on a local console? Also if you kill the two AI players after the 2nd Tank in the finale (to prevent anyone being hit by a special infected) will that void it?