The schematic for the Ripper can be found in Chapter 5 behind the rotating orb that you must use stasis on to pull down the arms in order to shut it off and intiate Zero-G. Proceed to the next store and buy and equip it. Make sure not to replace the Line Gun as one of your weapons as you will shortly encounter a Lurker for Clean Cut. Once you make it into the main hall of the Church, after encountering two Lurkers, you will be attacked by a giant necromorph that tries stabbing you with a spike extending from it's mouth. After it climbs away, you will get attacked by a group of the Pack. Use the primary fire on them and continue holding down aim until the achievement pops.
I did this in the Church of Unitology. After you repeling the female Tripod, and The Pack show up, I just ran back to the entry corridor as it serves as chokepoint to funnel The Pack into. Bagged about 8 or 9 Pack kills with a single blade right there thanks to the chokepoint.
@Gamma175, The Tormentor is creature AFTER that church part, right after the gunship shows up. The creature in the church BEFORE The Pack is a female Tripod.
You don't need to fire the blade to get the achievement. Just let 4 enemies gather near you, then use primary fire to rotate the blade in front of you and chop them down. After failing 4 tries in the schools gym to line up 4 of them, I got it on my first try when not shooting the blade.
I got this with the ripper's melée. I killed 4 pack within 4 - 7 seconds of each other without actually firing the weapon. This will probably unlock if you just kill 4 necros in a row, I don't think the time is an issue, as long as you don't fire the blade.
I got this one in a wierd way, i had a little trouble with it. When you get to the end of the game, after Nicole enters your mind, shoot nicole the first time then the heart. When the ghostly creepy kids come out to attack shoot a saw blade with primary fire and spin around a couple times. Happens the first time every time.
use primary fire with the ripper on the toddler things (not crawlers) aim for head and it will pop. They always travel in packs so its pretty easy rather then lining up 4 and shooting a blade with alt fire