At the end of each round, do not kill the last zombie. Kill all of the others, but allow one to live. When it has entered the building, throw a grenade at it to blow off its legs. It should now crawl slowly on the ground, leaving you plenty of time to patch up any windows you need. The next round will not begin until you kill him. He is easy to evade. Use this opportunity to get ready for the next round. When you are ready, kill him and the round will end.