Ghosts usually go down pretty well if you use electricity based stun weapons (stun rod/prod, taser, etc). They´re only stunned then though, so better just pile them all up and keep under guard while you hunt the rest down.
Otherwise massive amounts of conventional dmg may stun it (at least as far as i remember), but that really need to be massive amounts. Like 10 full salvos minigun or such.
Not sure if you can kill them, but don't need to.