[Kobal2]

Magic Duel is unique in that it's based on straight up astral skill. No boosters, no gems, no PotS/LotNS, no communion, no nothing.

Other S spells are just like those any other school: if the spell description has + or - next to a figure, it means higher level in Astral means better scores or lower fatigue, and in that case boosters and buffs do count.

Increased skill in the path required to cast the spell does make them harder to resist, specifically you get one point of penetration for every 2 points of S magic above requirements. Not sure if it's rounded up or down. Assume down. And boosters do count here, as do communions etc...
The full formula is: 12+DRN+skill/2 - MR+DRN+skill/2 ; caster wins ties.
In other news, today I learned that having skill in one path made you resist spells of that path more easily. Intuitive perhaps, but I didn't know that.

Spell Focus gives Penetration 1, which means that you get to substract one point of the victim's MR before they roll to resist.