I did this once, basing the tags off of an attack that appeared in Zeph's Heroes of Haxors. I don't remember the specifics, except that there's some sort of initiator attack that promotes into the group of arrows. You vary where the arrows fall by giving them a random velocity range. I believe the attack appears in Heroes of Haxors 2 as well, which can be dled from the Tain, so you could try extracting that.
If I understand what you're asking, you just need an iniator attack and a projectile group of arrows. I'd suggest the using the big myrkridia skull attack as the basis for your initiator attack and then modifying a dupicate of the flying skulls projectile group to make them arrows.
Okay, I did EXACTLY that. However ... for some reason my arrows are not spread out like the skulls are. I can't figure out WHY! The archer shoots like six arrows, but they're all in a tight group right next to each other. There is no spread.
Ok, try modifying the random initial velocity of the arrow projectile in the physics section of your arrow projectile tag (not the group tag). That should cause them to spread as they move at slightly different speeds. For reference, the myrk giant skulls have a random initial velocity of 0.035
Figured it out. There is something in the original single projectile tag. Inside the projectile group tag, when I changed the projectile from "myrkridia (large) skull" to "bowman arrow," that's when the archer started shooting projectiles in a straight trajectory. I went and created a new singular projectile based off the original bowman arrow and copied over a lot of the skull's data. One thing of *particularly important* note is that the spread of the attack is defined inside the projectile tag under the contrail menu by the "frequency change per tick" number.