I am looking at CR's and stats for an upcoming boss fight for my level 2 players. The party consists of a bard using the harbinger variant acting as a debuffer, a cleric who primarily functions as a healer with some benefits from the war domain, a paladin who essentially absorbs damage but does not deal much out, a ranger who is the primary damage dealer and also semi-tank, and a druid who acts as a secondary spellcaster and does a bit of the damage output. All but the bard are very vanilla, new players who used only the core player handbook feats and none of them have min/maxed or optimized.
The idea is to put them up against a black dragon in its lair with various cover available via stalagmites. In addition the party has one scroll of protection from energy (but no-one able to cast it without a use magic check) and one potion of resist energy. The cleric has a tower shield (which he could use for total cover against a breath attack) but is not proficient with it. The idea for the boss fight would be very climactic, evil monologue followed by a long drawn out fight. I would likely include a gong or other warning bell that summoned minions to the fray (low CR kobold bards and clerics who would buff and heal the dragon if it appeared the battle would be over too quickly. The idea here is to make a VERY difficult encounter in which one member of the party could likely die.
I know that at that level anything from a CR 3-6 is very difficult and 7+ is overpowering.
As a rule of thumb should a boss fight be in the low end of overpowering range?
I realize CR and EL are not entirely accurate all the time and a lot of factors play into the actual difficulty of an encounter but I am looking for a general rule of thumb.
My assumption is it should be on the low end of overpowering accounting for 5% of the overall campaign encounters. Or potentially the high end of very difficult.