Alternatively to EbonHawk01's solution, you can equip the weapon attachment "Statis Coating" on any of your firearms should your Statis module not be upgraded enough or if you are out of energy. This attachment will cause a statis effect upon the enemy. IGN indicates in their Dead Space 3 Wiki that "[the] stronger the projectile, the stronger the stasis effect." (http://ca.ign.com/wikis/dead-space-3/Stasis_Coating?objectid...
Performing the final shot on an enemy with this attachement equipped will count towards the Slow Mo achievement.
Much like EbonHawk01, I've obtained the achievement between Chapter 9-10 by simply shooting enemies with this attachment equiped on my Evangeliser. I used both aspects of the Evangeliser (Machine gun and Shotgun) with each weapon having a different statis duration. The Shotgun, upon inflicting the final blow, would cause the enemy to be pushed back in statis for more than a second until it finally drops on the ground.
Give it a try! If anyone could help me confirm this with other weapons (i.e. rockets, javelins, line gun, etc.), then that would be great!