In order to obtain the 'Pumpkin killer' achievement when it's not Halloween you to change your computer clock to October 31st.

On Windows 7 (might be the same on Windows 8 and Windows 10, as well):

- Exit game
- Go to the bottom right of your computer and left click on your computer clock
- Left click on 'Change date and time settings...'
- Click tab 'Date and Time'
- Left click 'Change date and time'
- Change it to October 31st, and press 'OK'
- Enter the game, go to any mission (on any difficulty) and follow along until a flaming pumpkin attacks you. It is random when it attacks you, so you may need to play a game or two.
- Once the pumpkin appears, it will throw flames at you. It isn't that difficult to defeat. You will obtain the achievement 'Pumpkin killer' once you killed it.

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