Here is the full list of all 12 I, Zombie achievements worth 1,000 gamerscore. It takes around 2-3 hours to unlock all of the achievements on Xbox One.
Collect 20 stars
Turn 10 soldiers into zombies
Lose a zombie
Die for the very first time
Turn a human into zombie
Finish all summer levels
Collect 40 stars
Collect 60 stars
Beat all summer levels with total time lower than 15 minutes
Beat all winter levels with total time lower than 8 minutes
Collect 90 stars
Finish all winter levels