This kind of airship will, once at operating altitude, be essentially be impossible to shoot down unless the enemy has a true SAM based defense (e.g. SA-11). SAM would have no problems locking on, as they tend to be driven by an active radar on the ground - I doubt you could hide something that big from radar in any useful way (although, I wonder if making it extra radar reflective might not actually work better since it would give the missile to large an area to aim for?).
Stingers have a useful ceiling of around 15,000 feet, and they're driven by infra-red, which means you probably wouldn't get a lock on.*
The only other thing that would work would be a proper flak gun at around 88mm. While there's a lot of those lying around Afghanistan, getting them in working order, manning them, and providing reliable ammo would all be very problematic. Remember that flak is only really useful if someone is manning it 24/7 - the ceiling might be enough, but the range is terrible.
* Of course, the problem with all this is that given the MOUNTAINS in Afghanistan, I wonder if there isn't a shoulder fired active radar missile available. The ceiling wouldn't have to be 20,000 feet, but rather 20,000 feet - the height of the mountain the defender is standing on.
Also, it looks gay.