You're just not going to get slept since you should have 100+ saves and spellbane.
Grand nocturne, and if they word, which they will, just chase them to their recall, or slaughter their horde.
They can't run from the fiend, their horde is useless against it since they can't rescue him from it, and no necro spells work on it, except maybe some damage ones. (not sure about vamp touch for example, though the healing it gives would be non-existant if it is tied to damage inflicted)
Tragic, pysch insight and a strong fiend will usually kill a necro. Especially if you are 51 and they are 47.
Undeads they do become less effective, but they can still do the job on anything except a lich really.

Though that was in the old days, and fiends are not quite as potent as they once were.
And if you pull it off once, the necro will for sure stock up on resist mental preps for the next time.