How to make an interesting bossfight last longer then 3 Rounds for a party of glass canons without a healer, that doesnt last 4. Scenario analysis.

So the the party (Lvl 11) I am DMing has a bit of a problem. And I would like to have other DMs oppionions on how they would have dealt with a specific situation we had the other day.

The Problems that I feel we have:

1. They dont have a support PC anymore because our Cleric left the table (about 9 Month ago) and right now they can punch realy hard but die pretty quickly (Monk, Sorcerer, Hexblade Warlock, Fighter).

2. Their approach to most encounters is "head through the wall"

3. They rarely prep., shop or often even loot enemys so they dont have many Item with them like healing potions etc.

Usualy they completly nuke any kind of bosses in 3 rounds max. so I wanted to give them some mechanics to make it more challenging without just doing something along the lines of raising AC to 25 or similar. To balance it I lowered the dmg, AC and HP of the BBEG herself so she would be more of a disruptor than a fighter.

The Scenario:
Group against BBEG, a bunch of Minions and two elite henchmen

The map was basicly a big rectangle with some Pillars on the sides

BBEG = Rouge/Bard mix that mostly buffs allies via a kind of inspiration mechanic and as a LA can make them hit in between their rounds. Also the BBEG had a hypnotic aura (Homebrew) that, as long as she moves 15ft., could hypnotise an enemy (via WIS save DC16). That aura was explained as some kind of dance and hypnotic tattoos, meaning no movement, no hypnosis. During hypnosis they couldnt move or attack for 1 Round exept if someone else frees them from it.

Elite Henchmen = two Assasins that were hiding and sneak attacked the PCs at the right time

Minions = just some 1HP Goons who had so low attack mods that they barely even hit anything at all. They were mostly used to pin down PCs

How it went:

The fighter, warlock and monk all charged to the right side of the map and got swarmed by minions. The Sorcerer was left behind and some minons charged him through the big opening in the middle/left.
The Hypnotic mechanic succeded a few times an cost the melee chars probably each at least one round during the fight.
2 Elites sneak attacked in round 3 and hit two of the players pretty hard

Sorcerer went down in round 3 and in the end was at two failed death saves.
Warlock and Monk went down shortly after the enemys damage dealers went in.
The Fighter was at maybe 5HP and still had 2 Minions and one ELite to go (The BBEG was at maybe 70% HP at this point).

So TPK was imminent....

The fighter had a single potion of superior healing and since the fighter, warlock and monk were all super close together, I allowed the fighter to throw it on the ground and the splash would heal all 3 (but with the 8d4+8 healing dividet evenly by 3).
Before that I made the BBEG lose a round through monologing because she was sure to win and the PCs are so weak and big plan blah blah....
The other two came back up, ignored the minions, charged the boss and completly nuked her within 2 rounds.

Before I allowed them to use the potion in that way they were very dissatisfied and thought the encounter was to hard. I agreed that there were to many minions and that the elites probably should have dealt less dmg per round (mostly because of the LA and inspiration of the BBEG), but remarked that their strategy and party comp realy makes things harder for themself than they have to be. They agreed on that one.

The whole thing is not a who is wrong here but more of a how to avoid this in the futur. Big bosses should be tough in my oppinion and give the players more to do then to just charge in and reduce an HP Pool to 0.

So what are you guys thoughts? How would you make an interesting Bossfight last longer than 3 rounds for a party of glasscanons that cant last longer then 4, without making it feel unfair?

(Btw. I realy dont want their PCs dead and so do they. After two years of playing a PK sould be more meaningfull then to just die to the villian of the week)