*Nidhogg...Does not take damage at all and doesn't even notice. That carapace is still hard as hell, and his pores are made of that carapace!*

*You and the Knowledge ring get another idea: Suffocation. Clog the pores, somehow, which will make Nidhogg die of asphyxiation. Due to the Knowledge Ring in it's non-Blackout, Omniscient state mentioning it, it might be how to actually kill it without a doubling up effect.*