an intersting aside about impact, which is used for its all stats down enfeeble, the spell is elemental and the duration of the stat down is based on elemental skill not enfeebling. While I almost exclusively use this spell on sch, rdm, and geo, this is the set i'd use on whm if i stole jezzs yagrush
ItemSet 328735
perception ring and enchanter earring+1 could be swapped in for irrwisch and strophadic. the elemental skill does increase duration, but does no good if the spell is resisted. A half resist will result in half duration on stat down.
personally, my lehbrailg+2 is augmented with MAB28 elemental skill 8 (first stone lucky), and i am in the process of feeding a twebuliij airlixirs on macc path.
I haven't tested the math on macc, but I was always under the impression that the equation for white magic enfeebles basically went as follows
200 + (0.93 * (skill-200)) + MND/2 + macc
substitute int for mind when doing black magic enfeebles. I'm not 100% as this was the equation way back, but I don't remember them changing it, just mob resistances.