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.
Collect 90 stars
Beat all summer levels with total time lower than 15 minutes
Beat all winter levels with total time lower than 8 minutes
Finish all winter levels
Finish the game
Collect 60 stars
Collect 40 stars
Play at least one community level
Collect 20 stars
Turn 10 soldier into zombies
Lose a zombie
Die for the very first time
Turn a human into zombie