i initially did my first playthrough with just the plasma cutter for
The One Gun achievement in Dead Space worth 80 pointsBeat the game using only the Plasma Cutter
and as such used a seperate save slot for the begining of each level for when i went for the weapons achievemnets. if you did too or have a save near level 12 then try the following...........
start level 12 - the building you go into on the left has a store and bench. sell everything you own except your med packs, ripper and ripper ammo. buy more ammo for the ripper and nodes to upgrade it using the bench. now start working your way through the level using the ripper and eventually you will come across 2 tentacle-wall-things. by this point you will probably have about 20-25 kills anyway so just kill a few of the constantly spawning bastards and 'pop' goes the achievment!
i used a combination of primary and secondary fire ( you fire the saw blade) modes. when the yellow-arm-sack people show up you definately want to keep them at distance!
you can use this method for all the other weapon 'cheeves. it, for me, seemed like the least hassle per enemy as there are loads of them in a short period of time!