I managed to get this with on Hard with my very first little sister that I adopted doing the first gather at Ryan Amusements (not the gather in the room with the turret. Also the same method I used in the 360 version). I sat crouched back to the left of the little sister on a pile of rubble in the corner near the door after I had set a few rivet traps on both sets of stairs. I noticed I had a few splicers come down the stairs, a couple from behind the vending machines, and a couple that dropped from the walkway above us and they landed right in front of me. Sitting in the corner with my Electrobolt and rivet gun I was easily able to pick them off one by one because they are all thuggish splicers so having some distance between you and them is best. One by one I hit them with the electrobolt and 1 rivet gun shot to the head and they were down.