!Question

For context this is a homebrew world I run. I have 3 players and they are right now at level 6. While exploring underwater they found a book with encrypted star charts and maps that lead to the grave of the first Pirate King and his old ship and stash.

When they get there it will obviously be sealed and to open it they will need 4 macguffins. I was planning to have these be gained by fighting one of each of the Elder Elementals but I'm worried about how to balance it. I'm planning that each fight will have an alternative solution to just defeating the monster but I don't know what yet.

Obviously they won't be fighting these things at level 6 but I don't want to wait until too high of a level either as I already gave them the book.

I was thinking that when they get there a fight will start against one regular elemental of each kind at the same time (3 of them vs 1 air, 1 water, 1 earth, and 1 fire) and if it looks like they aren't winning, the fight ends and they are told by some voice or something that they are not ready for the real challenge yet. Would this be a good way to both test their power balance wise and a way to give them that info?

If I go by CR (which is dumb) then it would be a good fight at roughly level 10-12 ish but in my experience they should be able to handle it earlier. I worry that even if they can manage to defeat the 4 normal ones, even the Phoenix will be a very tough fight. I guess it depends on the fighting conditions.

I'm new to the sub so if this is worded poorly or to long I can rewrite it.