Upper Spike: A level 49 character (last level before losing access to the Upper Spike)
Middle Spike: A level 79 character (last level before losing access to the Middle Spike - be careful as the creatures here are stronger than the level required, I've already died some times here with a level 79-)
The Queen of the Banshees Quest: a level 60+ with access to the last room, but once you reach the last room, you can't get out, you need to logout there otherwise it won't be possible to enter the room anymore. I also don't know if the last room is a no-logout area, this way is impossible to logout there and then you only have one try to get the fiendish. This area should be removed from the fiendish possibility list, I don't remember if Cipsoft already removed it, but if not, then it's worth to report to them if you ever encounter a fiendish there.
Treasure Hunt Quest and Eclipse: a level 35+ with access to the secret hole and access to the Eclipse mission - The Inquisition Quest. On both quests you can't finish their missions otherwise you will lose access to these places.
The Djinn War Quest: Two level 40+, one for each faction. But almost everyone has two characters to sell loot to both Djinn NPCs, so normally you can also use your main character for one faction and an alternative character for the other. There is no need to have a low level character for this specifically.
The Gravedigger quest also has some areas that you lose access after finishing the mission.
My honest opinion regarding fiendish hunting is to use your main or alternative high level character to find as many fiendish as possible but not overthinking it too much. If you find a fiendish spotted on an unreachable spot, just wait till it becames a normal monster and go after the next. Don't waste your time trying to cover all the map.