Curse should drop the target's resistances, and is better than Withering Curse, since it can not be resisted. You should also be able to cast Curse repeatedly to magnify the effect.
Are you quickly following up Curse with a non-physical based attack, to take advantage of the temporary resistance drop?
Do you have Know Creature? At level 5 it will display resistances, so you can check what effect the spells are having.
If you do not save before trying out skills (or it has been too long to just reload a previous save), you can use
iZakaroN's SaveEditor (or from
Kiya's site, with description) to unlearn them.