You can only shoot him with the turrets, sadly the antagonist's gun itself will deal no damage. It takes too long to reach a turret and get on it and deal any significant damage to bother with it, in my personal opinion, unless you just wipe out all 4 at once.
I see he got pretty close to dead there, but I also kept them busy fighting me and not the engineers. I don't trust the weakened AI to keep raiders busy if I sit on a turret.