Basically all you have to do is make sure you finish the game in under 5 hours.

This can be easy if you start on amateur and just simply run withouth picking up anything but gems and herbs if you really have to.

You will soon know if you have done this when you have finished the campaign's last chapter - 6:3.

If not, you can chapter select and redo any chapter you think took too long and shorten the time.

Once you finish the game under 5 hours you will unlock infinite RPG propelled grenades!