The worst ones have gotta be Amarok/Orthrus on light/dark day. Immunity to Kurayami, Light/Dark Threnodies, Flash, and Dispel... riiiiiiiight.

Also stuff like Bukhis being immune to Kurayami and Light Threnody.

And Gamayun being pretty much immune to all Threnodies + all debuffs including Kurayami and Flash.

There's a few other examples but it's mostly the Heroes NMs that resist light/dark at insane levels (it's hilarious that Ningishzida has higher resistance than Alfard btw) and harder NMs that resist their own element to an insurmountable degree.