State of Decay

I found this achievement to be easiest once I had made it to the church.

For this achievement you will need:

A gun (preferably a rifle or pistol)
A 'distraction' item (such as a kitchen timer, or talking doll)

Once you make it to the church, open your map and look for a small neighborhood to the left of the church (this is indicated by the multiple homes on your map, derp).

(You can pillage these homes in order to find a talking doll and a pistol if you don't already have those items!)

In the upper portion of the layout of various homes that I just spoke of (just a bit down and to the right of the gas station on the map), you will see a 'construction site' home, to the right of that construction site home is a House that has a side porch containing a barbecue grill. Go there.

Once you have gone to the home and found the grill, throw the talking doll next to it (select the doll or timer by using the LEFT or RIGHT D-pad, once selected, HOLD LT to aim, and then aim at the grill, use RB to throw the doll or timer) and the zombies within the house will come out. Once you have 3+ zombies near the propane tank on the grill, shoot it!

Achievement unlocked!

One of the gas stations from the main road out of the ranger station has a tank behind the gas pumps. I threw firecrackers next to the tank and when three of the zeds got close enough, I shot it with a rifle I had found.

For this achievement you're going to need a distraction I recommend firecrackers or a alarm clock. (I would take 3 just to be on the safe side). You will also need a gun.

Walk around the house's near the church until you find a BBQ grill and a propane take.

Walk up to it, select your distraction device by using the left and right button on the D-Pad and then use RB to use it.

Then run away and let the zombies go to the propane tank. If the're 3 or more zombies shoot the propane tank. If not, place another distraction down again and hopefully gather more zombies.

Once you shoot the propane tank under the grill and kill at least 3 zombies the achievement should pop.

(you can get guns with a sight on it but it is not really necessary)

Watch how I got it here.

Seemed impossible at first but as soon as i got it, lol very easy.
You need the following:

A Location with a tank (there are so many in the southern part/Town)
A loaded gun
A car
Some Fireworks (sometimes the zombies won't get distracted)
And a Zombiehorde

Lure them slowly with the car to a tank step out, throw the fireworks to the tank,
wait for the bait and shoot the tank.