Here is the full list of all 13 I, Zombie achievements. It takes around 1-2 hours to unlock all of the achievements on Windows.
Beat all winter levels with total time lower than 8 minutes
Collect 90 stars
Finish all winter levels
Beat all summer levels with total time lower than 15 minutes
Finish the game
Collect 60 stars
Collect 40 stars
Play at least one community level
Collect 20 stars
Turn 10 soldier into zombies
Die for the very first time
Lose a zombie
Turn a human into zombie