Slash Slash, Slashity Slash Achievement/Trophy
This can be unlocked once you reach the 'Old House' after dealing with Jack and finding the 3 dog heads. The start point in the following video is from the safe zone shack which is located on the other side from where you enter the house (A guide is below on how to reach the shack and get another achievement)
From the shack make your way through the halls and you will come across a cabinet that has a bunch of spiders on it. Start slashing away until they are all dead and you will unlock the Trophy/Achievement.