On the new Map Pack 2, Zombie Map, there is a really easy way to get the "Hammer Time" achievement on the very 1st level!

All you do is kill all the zombies, apart from one, making sure the last one is not in the room yet.

Then as it breaks down the window boards just keep pushing blue to rebuild it.

As it's the 1st level the zombie is slow taking down the boards so you can easily keep rebuilding them - keeping him outside.

Just keep a little distance from the window as he can still attack you and after about 5 minutes you will get the 10 point achievement.

Simple!