You could do something like the d20 modern autofire rules. Target a square with your arrow. Squares have AC 5, but the range is going to jack that way up. Allowing indirect fire in the 11-20 range increments, the range penalty will be from -20 to -38.
If you hit, you hit that square. If you miss, roll a grenade scatter to see what square you did hit. Whatever square you hit, the guy in that square gets a reflex save DC 15 to avoid damage (stepping out of the way or raising his shield), otherwise he takes damage. No crits are possible, obviously.