Here is the full list of all 54 1 Million Zombies achievements.
Destroy every Zombie on the Coastal Carnage Level
Destroy every Zombie on the Farmageddon Level
Destroy every Zombie on the Fairway Frenzy Level
Destroy every Zombie on the Western Wasteland Level
Destroy every Zombie on the Radiation Rampage Level
Destroy every Zombie on the Neon Nightmare Level
Destroy every Zombie on the Dockside Desolation Level
Destroy every Zombie on the Area 51¼ Apocalypse Level
Destroy all Destructible Items on this Level
Exterminate all Zombies across all Levels
Kill 100,000 Zombies
Kill 250,000 Zombies
Kill 500,000 Zombies
Kill 1 Million Zombies
Kill 10 Million Zombies
Destroy all Glass Houses on the Harvest of Horror level
Unlock the Harvester of Horror
Unlock the Doom Buggy
Unlock the Golf Cart
Unlock the Brute - Ute
Unlock the Slambulance
Unlock the Ice Cream Truck
Unlock the Experimental Military Vehicle
Unlock the Spider Vehicle
Unlock the Fire From Above Minigame
Unlock the Gunfight at the KO Corral Minigame
Unlock the Slice n Dice Minigame
Unlock the The Claw of Doom Minigame
Unlock the Whack a Zombie Minigame
Unlock the The Fast and the Dead Minigame
Unlock the Roulette of Death Minigame
Unlock the Street Cleaner Minigame
Aaayyyy, Jump the Shark
Unlock the Farmageddon Level
Unlock the Fairway Frenzy Level
Unlock the Western Wasteland Level
Unlock the Radiation Rampage Level
Unlock the Neon Nightmare Level
Unlock the Dockside Desolation Level
Unlock the Area 51¼ Apocalypse Level
Destroy all of the Tents on the Wild West
Destroy all the Light Poles on the Docks Level
Take out all of the Golf Flags
Score a Goal on the Nuke Level
Jump the Rollercoaster
Blow up all of the Propane tanks on the Area 51¼ Level
Kill 1000 Zombies with Chicken Bombs