Here is the full list of all 11 1 Million Zombies Demo achievements.
Kill 100,000 Zombies
Kill 250,000 Zombies
Kill 500,000 Zombies
Kill 1 Million Zombies
Destroy every Zombie on the Harvest of Horror Level
Destroy all Destructible Items on this Level
Kill 1000 Zombies with Chicken Bombs
Destroy all Glass Houses on the Harvest of Horror level
Unlock the Harvester of Horror
Unlock the Fire From Above Minigame
Kill 10 Million Zombies