This can easily be done with Barry in Episode 1. One of his starting weapons is the Magnum Python. It kills almost everything in one shot, and the bullets penetrate through multiple targets standing behind one another. Line up two enemies, and shoot them with the Magnum Python to get the "Line 'Em Up!" achievement. There are several enemies in the garage near the start of Barry's campaign. There are also some sneaking passages shortly thereafter where the enemies are standing still.