I haven't checked the amount of lightning resist they have, but if it is low enough, you might be able to remove them with a wand with lower resist charges. Conviction from infinity may also remove them. Can be used combined too.
Another way is to use the gavel of pain, and cast the amplify damage charges on it, and help your merc out with jab, dealing physical damage.
You can also just kill those that aren't lightning immune and just pick the drops.