How to: script Artificial Intelligence

[YinYin]

you will need to run a for loop inside the id function that goes through all object slots that can contain hostile projectiles and check them all through (distance/direction/speed, team and type)
store the object number if the conditions for it being threatening are met - overwrite it if you find a closer/more threatening object in a later loop

once the loop is done reload the most threatening object (last object number stored) - voila, target changed

keep in mind you will also need to still apply conditions for normal character targeting (and possibly also item picking if you want your char to do that)