Kobold Avenger
Legend
Yeah, he would need to be hit by a power that does both. I can't think of any off the top of my head, but there's probably some out there (or will be at some point).
Cold and radiant damage, along with fear are a thing for Starpact Warlocks. I'm sure they have some powers that do both.
And Mordain is a unique villain, I'm sure he's meant to not have many things to be weak against.