The Walking Dead

First go and look at the bell tower and talk to Omid. You suggest looking for a way to get across to the bell tower. Go all the way to the right and you will find a ladder. Take it and use it on the bell tower to get across.
When you are part way across, the ladder will break and you will have to quickly jump for it.
Ring the bell, and then jump back to the hospital, grabbing the edge. You will then head back to the house.

Once inside you will assign tasks te everyone, but Omid will notice that the front doors are wide open. You need to find a meat cleaver which is in one of the drawers in the kitchen. Use it to cut off the arms of all the walkers.
The walkers will break into the house.
Upstairs you will shoot as many as you can before running out of bullets.

You get the achievment whe you go into the attic.