[thejeff]

Is that 17MR for the spiders with riders, or after the rider dies?

Because I thought the Spiders themselves were down at the animal level, 5 or so, and I don't see how Iron Will and a bless gets them to 17.
Before the riders die they're not bad for MR, but the riders are fairly fragile despite the high protection, so you usually wind up with a good number of Spiders, who then die in droves to any MR-based attacks