For this you'll need to kill one of those baddies of the night. Zombies, skeleton archers, and creepers are your targets for this achievement. You'll have to wait until they spawn at night and you cannot be on peaceful difficulty. To make this guide easier be sure to have a sword on you, as they do alot of damage on MOB's. All you have to do is go toe-to-toe with one of these monsters and if you win you'll pop this 20G achievement!