I dont know if anyone else has found this or if anyone else knows this, but you can get it on the first round, all you need is one zombie left and he has to be outside the window, what you do is while he is taking off the boards instead of standing close to the window, go to the window and slowly back up until the "press x to repair or watever disappeers" then slowly go forward till it popps back up then freeze, dont move and you can bored up the window while the zombie is taking the boards off, but he wont go to reach for you, so u wont die, it may take about 5 to 10 mins, but its the easiest way to get the achievment Smile

hope this helps

if no one else has found this, then plz give me credit if u re-post this ^.^ and if someone else found this before me i did not know about it and i give them the credit ^.^ thank you