One thing I hadn't realized before is that a spider queen with the defence reduction from torches is actually pretty much the perfect target for rampage. Not relevant to the one you kill in spider infestation, obviously, but its great against the one in the small mine. You can deal so much damage in one turn you can skip straight through the part where he summons allies.
I'm wondering if the demon core, if you have the double atk buff from Charlotte actually has a low enough defence too, relative to your boosted attack, that rampage deals massive damage.