If you're trying to avoid lightning immunes, go with the Arcane Sanctuary. Nothing there (except those occasional unique monsters and the lightning 'towers') is lightning immune. And there are tons of those specters/ghosts, too. The only drawback is that if you kill one of those ghosts off of the 'path' (which is *very* likely, considering the 'path' is mostly very narrow), they can't drop anything.
But Arcane is the best IMO, just because of the no LI's, and the three areas of 3+ chests which usually give you lots of drops. I got an Um from the Arcane just the other day.