Note: The Anti-bio beam requires the line to be exactly on top of the crew member to damage them, in case you might think you only need to hit the rooms they are in.
Not true. You only need to hit the cell that they occupy, which are the little grid blocks that rooms are divided into. This, with careful aiming, makes it possible to hit three guys in the same 4x4 room despite the fact the are arranged in an "L" formation.
One way to make getting this achievement easier is to hit boarders that have just warped back to their ship.