You can easily get this achievement during the first battle in Chapter 12 using Five Barrel Barrage. Basically, I used Wrath of Katath on an enemy surrounded by other enemies, and then Five Barrel Barrage (great idea if you didn't get that achievement in Chapter 11), which took out 5 enemies affected by Wrath of Katath. I reloaded and got four enemies. I reloaded again and got the last enemy I needed.