The spell Vilano’s Sling introduced in Houses of Hermes: Societies book many years ago defines how magic can be dangerous to any creature, even those normally protected by magical resistance.
Here is a modification of Vilano’s Sling to cover a wide area of effect, with one hundred projectiles.
The Chaotic Storm of Stone
Rego Terram 35, R: Voice, D: Momentary, T: Group
The spell animates up to 100 pebble or bullet sized stones, and causes them to fly about randomly, in an area of effect shape determined by the caster (an arc, square, etc).
The area uses the caster’s voice as a practical size limit, typically a 20 pace radius at round the caster. The stones are flung about by the spell in a manner akin to The Invisible Sling of Vilano.
The number of potential targets is limited by the volume of stones present, each being targeted by a stone in the storm suffers +10 damage.
The spell design allows the caster to never be directly targeted by any of the stones, so they may stand unharmed.
(Base 10, +2 Voice, +2 Group, +1 size of Group)
Then a reworking of Vilano’s Sling, to grossly increase the damage inflicted by the rock or bullet. The reworking is based upon the Rego Terram guidelines in HoH:S p. 38 which has examples for level 5 inflicting +5, level 10 for +10, and level 15 for +15 damage.
The Vicious Projectile
Rego Terram 25, R: Touch, D: Momentary, T: Individual
This effect hurls a stone in a manner identical to Invisible Sling of Vilano, with the force of the projectile increased to inflict +20 damage. The stone is accurate to 20 paces and requires a successful Finesse roll to hit the target. As the stone is flung by spell rather than guided, the effect is not influenced by Magic Resistance.
(Base 20, +1 Touch)
This was the simplest approach. A more complex rationale might also increase the range for which the spell can still inflict damage, due to the fact that the projectile probably is still moving quickly after 20 paces, and should probably decrease it’s damage incrementally. i.e. +20 damage out to 20 paces, the +15 to 30 paces, then +10 at 35 paces, then +5 at 40 paces. I cannot think of other spells which use this kind of rationale; a sympathetic troupe may be open to it.
These and many more custom spells can be found in the New Spells for Ars Magica page.