Here is a part of how i frame things with magic vs multiple opponent like repel undead, meteor shower, fear etc.
Group simple contest and extended contest
4.0 Some spells affect multiple opponents. When one of these spells is cast as part of a simple group test, the combat is divided into three tests: one to determine whether the spell succeeds or fails, the other to measure the result of attacks made by other members of the side to which the caster belongs. The points gained or lost by the caster will be added or subtracted from the result of another group test between the opposing camp and the companions of the caster, if applicable. The final outcome will be determined by the sum of these two single group events. The skill used by the opposing camp depends on what the latter wants to achieve, and the consequence of his victory if he wins follows. It may be that this skill is luck and that the consequence of passing the test is only that it is not affected by the spell. Since all of the Beligerents present act at the same time in the Single Group Event, one may wonder which of the two trials should be done first between that of the caster and that of the other Beligerents. It doesn't matter, but initiative always goes to the highest skill score.
4.1 In an extended group event, the caster rolls the dice against each opponent present in the area of effect. If he succeeds the test against opponents, the latter subtract a total of resolution points equal to the success of the test. Opponents may oppose an offensive skill. If this is the case, the caster will undergo the average rounded to the whole higher of the points of resolution marked by each of the adversaries who will win the test. If the sum of the points scored by four opponents is two, the average points per opponent is 0.5. No point is therefore retained against the caster. If resolving points are retained, they count for all opponents who make successful use of the offensive skill they oppose to the caster. But the points in question are floating. They will count at the end of the confrontation only for the first opponent who will total 5 points of resolution against the caster. Floating points scored by other opponents will not count in calculating injuries. This is so as not to unduly penalize the caster with an unfair total of injuries. Otherwise, opponents might as much benefit from the caster's casting a spell as the caster himself and that wouldn't make any sense.