[Imp]

With that situation I would have most likely just pulled out & left him to it, let him shoot at ghosts.
Look for a good place to take him on or especially if you have mobility while those units are advancing slowly & out of the the battle for a while go attack something else with your superior numbers.

If the situation permitted I might have tried flanking round behind him & plotting all spare artillery on his position. I would then just shoot the runners.

Generally I z fire with support equipment MGs etc rather than squads as it gives away so much info.
Squads may do the odd z fire in woods as back up to artillery but not much its pretty inaccurate in this situation.
Only other time I tend to z fire squads is to suppress a hex if I know the fire came from there but I cant see the unit.