I think you may have gotten Withstand wrong. If the result of the spell depends on Potency then the effect will be partially reduced by Withstand even if it isn't fully cancelled out. Example: A spell with Potency 5 imposes a negative modifier equal to Potency on the target's dice-roll, so it would be a -5 modifier. However, this spell is Withstood by the target's Composure, which happens to be 3. This reduces the Potency of the spell to 2 (5-3=2) and so the modifier is also reduced to -2.
Spellcasting is never mundane. You're using your link to the Supernal to change the Phenomenal world. The word you were looking for was ordinary probably.
