Archers vs outsiders split from unfairly powerful monsters

[ColorBlindNinja]

The CR of a Pit Fiend is not under dispute---it's stated very explicitly in the Pit Fiend stat block.

Are you trying to prove that a Fighter is inadequate? If so, you must pay attention to Encounter Level because guidance around encounter difficulty is provided w.r.t. Encounter Level.

No, if a level 20 Fighter can't handle a CR 20 monster that uses its abilities intelligently, the Fighter is deficient.

It is a lethal problem for the Pit Fiend in the long run because for every mummy-day, the Pit Fiend has a 5% chance of failing the save making it easily attacked by the mummy and eventually contracting Mummy Rot which is lethal to a Pit Fiend. As a consequence, living with mummies is a suicidally paranoid strategy.

Why are the Mummies attacking the Pit Fiend? He has Diplomacy, and he's immune for 24 hours whether he makes the save or not.

A Planetar is certainly a good challenge, but is there a non-good equivalent? I think that's reasonable in the sense that many/most parties don't fight good opponents in real games.

Evil parties don't exist? Misunderstandings don't occur between good creatures?