For this achievement you must create the turbine, which is composed of the parts in the bus stop at the start (one against the sign in the middle, one beside the bookcase next to the M14, and one on the bench next to the power needed door). Just place the generator at each door of this sort at each stop (bus stop, diner, farm, city, power room). One will not have a part; the other four will. Place the ladder on the back driver-side of the bus, the grill on the front, the emergency ladder on the emergency exit on the inside. If you wait until a zombie destroys the ceiling, you can skip the fourth part. If not, there is a fourth part to attach on the emergency exit.

After Noriega gives you the sniper rifle on the rooftop, shoot your target in the knee twice instead of killing him to see a different ending.

You can kill the giant a number of ways but he can be resistant to some guns. Effective guns include the Acid gat, Death machine, Lsat (with headshots), or a LMG with the Deadshot Daiquiri perk doing headshots. These will usually kill him without wasting too much ammo.