Each type of enemy has an english word ranging from 3-6 letters in length. The letters are shuffled randomly.
For example happy is shown as hpapy
Use the buttons on the bottom of the screen to type the word that is shown by the selected enemy. You need to tap each button in the correct order to kill an enemy. As you type, the order of the letters will change reflecting the order you typed them in.
Pressing "h" results:
hpapy
Pressing "a" results:
happy
In this case you only needed to enter "ha" since the rest of the word was already matching.
Enemies with shorter words are usually faster.
The target is automatically selected, but you can switch targets by tapping on different enemies. You might need to do that if one of the zomibes is approaching faster.