[thejeff]

Also many of the Black Sorcerers can cast Twiceborn, all with a Skull Staff. That helps mitigate the age problem.

As said, excellent battle magic: Blade wind, Magma Eruption, Flaming Arrows.

Black Sorcerers with Fire Shield & Invulnerability make good thugs. That's what their spider form is for. If they had a change shape command they'd be brutal.

As for units, Spider Knights are pretty robust and once you've got a bunch of them, the web becomes surprisingly effective.
Dual bless Black Hunters are also impressive. Fire/Earth works well for the Hunters, Fire/Nature once the rider dies.
I wouldn't worry about arrows, they've got either good protection or lots of hp. The low MR of the spider form is their biggest problem. They weren't affordable in Dom2, but with the extra money in Dom3...
Blessed Hunter Lords make good thugs as well. The spider form of the Black Sorcerers is sacred too, but I've never been able to get them blessed reliably.