I couldn't get the other solution to work as the soldiers killed each other almost every time and I tried other sections and nothing worked. As for the super shotgun, you should have it by map 10. In case you don't, as per Apache117X's solution, "The Super shotgun can be located in Level 2: Underhalls", credit to him for that.
When you start Map 10, save at the beginning of the map so you can retry in case you miss your shot. Once you've done that, there is a super easy spot to get the achievement. Run into the first area, try and pool the soldiers, exit the area and then blast them with the double-barrel shotgun in the entrance. Bloop, achievement unlocked. If you don't get it, reload and try again. The video below shows it. Hope it helps.
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