The question still remains unanswered, does anyone know the decrease in combat damage? I recall from this past BWE when I cast it on a Stone Elemental that was beating on me his damage went from aprox. 15 to around 3.
I can't remember exactly where but I'm 99% sure that weakness reduces the damage dealt in melee by 66%. Therefore a person that used to do 10 damage would now do 3 damage insted. It's a very powerful counter-condition against warriors.