The funny thing is, you all keep missing the point and essentially repeating what I keep saying. So let me put this in a context that you can understand.
Demonlords are very tough fights when ran right, but usually winnable. It can be an unwinnable situation if fought on their plane in their lair with their minions, but if summoned and having to fight alone, it should be pretty winnable.
Archdevils if ran right are pretty unwinnable. Not because they have broken abilities, but because they have teleport and regeneration. This allows you to be more tactical in combat, which works for devils since this is how they fight anyway. In a situation where the arch devil has to fight alone, he can use hit and run tactics, regenerate and come back never giving the players the ability to rest, which will cause them to use up their resources. Zariel is a perfect example of in normal conditions she would be unbeatable without having broken abilities.
