Thinking about taking the dive...

[Yogi_Baar]

Also, I don't think a necro wand with charges of AD on it would be such a viable option for melee characters on the switch as a solution for PIs, if it worked like LR and would only function at 20% efficiency against immune monsters. Feel free to correct me, though.

As your test indicated, it does function at only 20% efficiency against PIs. The other, or, indeed, main reason why melee characters don't get such wands, is that they don't in fact exist. IIRC, charges of AD only exist on some (and precious few) unique items, like the Gavel of Pain.

Because I know that only 1/5 of the damage should get through, I probably got lucky on the side of his damage, or on the side of low hitpoints of the wraiths.

Another explanation is that he used Jab. They have a two-hit Jab skill, IIRC. I'd check the AB mercenary resource to be sure but the site is down at the moment.