Throwing MULTIPLE rocks

Suppose there is a spell with Target: Group, that throws rocks like Sling of Vilano (from HoHS) - an aimed spell, hurling the stones, rather than directing them with magic like Wielding the Invisible Sling. Depending on the rock's size, it can throw ten, a hundred, a thousand stones in one spell. How to manage that mechanically ? How many aiming rolls to make ? With what bonuses ? And for what damage ?

I'm inclined to say that, for simplicity, the rocks are thrown as a group. So you can aim them at a single target with no real penalties, making a single aiming roll, and as many hit as makes sense. A single rock does the spell's basic damage, and extra hits mean only slight extra damage (perhaps +5 for the next rock, no more damage at all past three or four hits). If you want to finesse the targeting more, throwing, say, half the rocks at one target and the rest at another, then you make one aiming roll per target with -3 per extra target.

But this is just a gut feeling. Anyone got any comments / better ideas ?

same damage but give +6 bonus to Finesse roll to hit seems right to me; more likely that something hits

We discussed that extensively in the Héviz saga. Our conclusion was that an aiming bonus is fine, and so is making the spell an area effect one (there are plenty of those in the books). But allowing multiple damage rolls on the same target, or allowing the caster to choose a certain number of targets without making the spell area of effect, would be unbalancing.

I think that was the general conclusion. YMMV.