I made it to wave 50 by going inside the building that has a mortar on the roof. You can hide behind the bar wich allows you to unlimitedly pick up bullets for your hand gun.
If you have a shield place it in the front door so no one can get in. If you have any bombs place it where the bar has openings so if they get in, they get blown away anyway.
Tip: Inviting one friend makes getting this archivement a piece of cake. (Make sure that you reached multiplayer xp level 50 to get this archivement). Good Luck!