From my personal experience with her, Elise loves magic-pen and tanky items. Guise -> Liandry's, Abyssal, Frozen Heart, that sort of thing.
EDIT: You know, while I'm here, I have a question: how is the new Malady on her?
Unless I'm missing something big I don't think building AS will ever benefit Elise more thank tanky ap items. Most of her damage comes from human form, and once you switch into spider to DPS malady's proc won't be very effective as the enemy/enemy team should be dead or dying by the time you're switching back to human. Or close enough that it's not effective.