They Are Billions Calliope Build ((install)) -

Shoot a building to aggro the swarm, run backward into an open area, and pick off the fast runners first.

Never stand still with Calliope when facing a crowd. Fire a shot, click backward to step away, and immediately press your attack hotkey (Default: A or Q depending on your control scheme) to fire again. This minimizes the distance the infected can close between her shots. Utilize Chokepoints and Barricades they are billions calliope build