Beast Master Ranger balance

Sage Advice: Master Jeremy is right, it depends on context and your DM campaign.
If your adventure is in a multi level mega dungeon could be an hard task for Rangers, Druids and, if dungeon is very narrow, casters that can’t use their area effects/long distance spells.
But the problem could be similar in a city, where is more important to be sneaky or charismatic : area effect caster or Barbarians could be very cumbersome.

So my advice is to talk with your DM before campaign start and decide together wich classes could be better for all of you.

Evil comes in many forms: The 4 Princes of the Apocalypse

The King of Dungeon Masters, Chris Perkins, gives a blast from the past, something from an ancient tome called Fiend Folio: here are described the four Elemental Princes of Evil that will be in the upcoming campaign adventure Princes of the Apocalypse.

Master Chris is giving suggest me another thought : Princes of the Apocalypse adventure narrates about 4 Elemental Evil Prophets that are linked with their Princes of Evil, here their connection:

If you want to dig in the knowledge about The Four Elemental Princes of Evil you need to search their alias: The Archomental.
In Planescape settings there’s the Planescape Monstrous Compendium Appendix III that well describes all of them and their relationship with the Tharizdun/The Dark God/Elder Elemental God, they are powerful but not demigod, they grant spell to their followers and yes, they can be slain.

If you want to play now a good prequel of Prince of the Apocalypse you can play Egg of the Phoenix adventure module where player characters must recover a powerful artifact and save an entire land from destruction by the Princes of the Elemental Evil.

Other references about four Princes are

There’s only an unanswered question:  where’s the fifth prince? The real son of the Elder Elemental God, Cryonax, Prince of Evil Cold Creatures???

Dungeons & Dragons Classics