It depends a lot on the archetype you’re going for. Offense can usually get away with just AV Magearna and a Water resist like Serperior or Rotom-W. If you’re really worried though, you can try fitting something like Choice Band Tapu Bulu or even Mega Gyarados. On teams a bit bulkier though, something like Toxapex, SpDef Tapu Bulu, or even Tangrowth is fine.Thanks so much for the quick reply! Mega-Scizor seems like a great option considering most Kyurem-Blacks that I've seen have been running Earth Power lately to deal with the ever-present Heatran. AV Magearna is an interesting option as well considering it's a lot more specially bulky than Tangrowth. The only thing I would worry about with running AV Magearna over Tangrowth is an increased weakness to Ash-Greninja.
What mons do you think might partner well with AV Mage to act as another soft-check to Ash-Gren?