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 ?