Here is how I fought the last battle. It took me two tries to get it, but in the end no one died:

Spell Strategy
Mage: Load up on mirror image
Cleric/Druid: All heal spells including 2 heals each.

Potion strategy:
Fighters: two speed for non boot fighter two giant strength each 2 extra heal each. Potion of regen for booted fighter
Mage: 5 extra heal, 5 regular heal, 2 speed
Clerics: 5 regular heal, 2 magic blocking, 2 fire resistance each
Thief: Heal, speed, mastery thievery

Weapon Strategy;
Fighters: No two handed swords. Each had -8 AC
Thief; Bow with lots of piercing arrows.

On first move hit pause.Thief retreats west avoiding golem with find trap on. Mage retreats south then casts mirror image and then drinks speed potion. Fighters advance to Demon followed by clerics. Upon arrival each drinks giant potion. Non speeded fighter then drinks speed potion. they start to hack. Clerics immediately cast cure medium wounds on targeted fighter. at

Thief moves around and identifies traps. When demon teleports fighters and clerics reengage avoiding traps.

Mage is key: I discovered that Golems target mages first then clerics then druids in my first try. the mage stays along the back wall speeded and mirrored moving east to west. He lets the golems reach him and elmininate one mirror. then he races. When mirror runs out he recasts and then drinks heal potions. this keeps golems from impacting main battle.

Battle ends with no player with less then 30 hp.

What did everyone else do?

I turned my mage and cleric invisible, and sent my paladin, fighter, ftr/thief, and ranger straight into Belhifet's face and beat the stuffing out of him. Game over.

like your approach. tried it on my second go and the golems ate into my fighters to fast to heal. Did not have the same fighter power you had

I think this "brute force" method worked because I had two fighters with five proficiencies, one wielding the long sword of action +4. He was a real buzzsaw. They all had armor classes in the -8 to -12 range, too.

I just put my mage in Otiluke's Resilient Shield. The golems keep attacking the Shield, even though they can do no damage. This takes the golems (and the mage) out of the battle. The rest of the party beat up on Belhifet with melee weapons, following him around as he teleported. No potions, no magical enhancements. And no healing needed. And yes, everyone still finished with 30 HP or more.

My tank dwarf had the Boots of Speed, so he was always the first to engage Belhifet after he teleported and therefore the one Belhifet would target. And Belhifet couldn't hit him. The only real problems were the fire damage from when Belhifet teleported and some damage from the cloudkill when he teleported into it (and attacked the invulnerable mage).