One can counter a spell with spontaneous magic; matching half its level with an appropriate spontaneous fast cast defense can generally protect the caster or another person (Core book, p.83).
What about countering multi-cast (formulaic) spells? Spontaneous magic can't be multi-cast. I would require the spontaneous magic effect to be increased by the corresponding modifiers for the Group target: i.e. +1 magnitude to defend against up to 10 "copies" of a spell, +2 for up to a 100 etc. (ok, it's an extremely unlikely "etc.").
Could this reasonably include defending against multiple (distinct) spells? I would say yes, with exactly the same rules, though of course it's tricky to find a defense suitable to stop a PeCo, a CrIg, and a ReMe combination - so that modifiers for complexity and requisites should require correspondingly higher defenses.
Do you think it's reasonable? Would you do it differently?
I'd like to remind everyone that the one half level guideline is for use when common sense doesn't give a clear answer. If the sponted spell could reasonably stop the multi cast spell it should if it doesn't it shouldn't. If it's unclear then we should start looking at the half spell level + adjustment for group.