Pull out your grav gun, hold down mouse2 and aim at the nearest manhack. It'll eventually fly straight into your grasp, at which point you aim at the nearest wall or floor and fire it. The manhack splatters instantly, and you can do this quickly enough to kill 2 or so per second if they are close enough!
Also, while you can't pick headcrabs up, you can still blast em with the primary fire of your grav gun, which saves on ammo.
Also, while you can't pick headcrabs up, you can still blast em with the primary fire of your grav gun, which saves on ammo.