State of Decay

You can get this almost straightaway from starting the game.

Progress the story forward until you are tasked to raid a Vets Surgery for supplies, your partner states no cars, so sneak back out and stick to the left-hand side of the road near the mountain, and make your way back to the target.

As you move along, you will pass by 2 or 3 Horde's, make a mental note of their location.

Now, finish the mission - head back outside and get a car. Head back to the position and they should still be very close.

Mow them down, and then simply drive around looking for the 7-8 more that you need.
When you kill the last Horde Zombie member, a little text confirms in the top-right, so make sure you run them all over until that flashes up.

I got 7 in that main town area, and I was exploring for more when I got killed.

I spawned as a new character, retrieved my belongings and then killed the last Horde to unlock the Cheevo.

Therefore, I can confirm it can be done across different characters!

I found that the easiest way to do this was to run over the hordes in the pick up truck that you get outside the church. A notification will pop up on the screen at the top right hand side telling you that the hoard was killed.

Simply do this 10 times within 1 in game day :).

Not far into the game, you have to climb two towers and point out hordes so that the army Sgt. can nuke them. This should get you halfway, so is easier than risking your neck right at the beginning.

Simply finish off a few more, and you're done. I got the achievement by accident thanks to this mission, so don't waste time grinding until then!