FATAL FRAME: Maiden of Black Water

Although there are several areas in the game where you could farm this achievement, the best place would be during the final boss fight in the last drop in either Easy or Normal difficulty. Try to get around 20-30 Herbal Medicine and around 1-2 Purifying Embers before starting the final fight just to be on the safe side.

While fighting Ose Kurosawa, play normally until you get to her second state (where the two of you are above water). Wait for her to spawn in random ghosts to taint you with the Black-Water or let her taint you yourself Once infected, stand close enough to Ose so that she grabs you. You'll now be in a first person point of view with groups of ghosts flying at you. They'll go down with one well-timed shot as long as you get all the ghosts in the frame each time. You'll eventually escape, but you'll need to be grabbed a few more times to until you've killed enough of those ghosts to unlock the achievement. Use the Herbal Medicine to maintain your health and don't hesitate to use a Purifying Ember if you need to.