State of Decay

To get this achievement you will have to sacrifice a character by blowing them up when they are downed. Death is permanent, unless you back up your save on another device. You could also shut off your xbox after getting the achievement before the game saves. If you don't feel like doing this, choose a useless survivor to sacrifice.
First you'll need either a grenade, firebomb, petrol bomb, or some other throwable explosive. I've heard of people having trouble popping the achievement with molotovs, so you might want to wait until you have a grenade.
You might have luck finding a hand grenade in the military stashes up north at the fairgrounds. Because this is Breakdown mode, the gate will already be blown allowing access to this area. Firebombs are commonly found on wine shelves in houses. Petrol bombs can be aquired by breaking fuel resources.
I would remove all items except the explosive from your character since they are going to die. Equip the explosive in your item slot next to the minimap. Run into zombie territory. Hold RT and press X to taunt and have all nearby zombies come at you. Let them down you. Instead of pressing A to recover, let them drain your health. Spam RB when your health gets low and when you're about to get torn apart. Your character should drop the explosive by them and blow up, taking out all nearby zombies and popping the achievement.