Back Off Mrs.B! Achievement/Trophy
This will be a duel guide for both the 'Back Off Mrs. B!' achievement/trophy but also the 'Less Is More' Achievement/Trophy also.
During the Old House missions you will need to go into the basement to find the crank park, this is where you will find and talk to Mia. After this encounter make your way back up the stairs and through the door immediately right of you to find Marguerite taunting you and telling you to leave. Kill the spiders on the door (You can use your knife to get the Slash Slash Slashity Slash Achievement/Trophy right here, see below link for a detailed guide) and go towards the far left door in this room. Marge will be right around the corner so pull out your shotgun and start shooting the swarms, these count as double kills so that will unlock the 'Less Is More' achievement/trophy. Now continue to shoot Marge until she decides to grab you and throws you to the ground to get the 'Back Off, Mrs. B!' Achievement/Trophy.
Here is a guide to the achievements:
Here is a guide to Slash Slash Slashity, Slash!: