Hey, the new, yound fellow ofpec-members will blame me, but I'm an oldskul, elder guy, so here it is my solution:
It not uses the Arm,A2's arty module, it just create shells in the air above the city and fall down...
You can change the radius and the timings and the shell-type. When I use this script ON player I don't like the heavy shells, because it easily kill the player and not a big fun. But if you need to destroy that town, you can use heavier shells...
_target = _this select 0
_radius = _this select 1
_tx = getPos _target select 0
_tz = getPos _target select 1
noneedarty = false
#loop
_shell = "G_40mm_HE" createVehicle [_tx+(Random (2*_radius))-_radius,_ty+(Random (2*_radius))-_radius,80];
~(Random 5)+5
?(noneedarty): exit
goto "loop"
Copy this text into a textfile and name it: mortar.sqs
Then you can call eg: [townGameLogic, 80] exec "mortar.sqs"
Where townGameLogic is the name of a GameLogic in the center of your target town and 80 is the radius of strike.
If the strike is not heavy enough,
you can change the "G_40mm_HE" for "Sh_125_HE"...
If the strike is not intense enough, you can change "~(Random 5)+5" to "~(Random 3)+2"