Angoth's Arcane Sanctum

Just a boss fight I designed for my current D&D campaign. Do you think it would be challenging enough for an 8th level party?

Attached picture caption:
1. Door, closes when the fight starts until phase 2
2. Door, closes when the fight starts until the end of the fight
Brown rectangle. Angoth's desk
Red stripes. Gullies that spew fire that engulfs an entire section (The sections are divided by the stripes)
L. indicated a level that can shut down the Fire Gully Defence System.

Phase 1:
Loremaster Angoth will be reading at his desk.
This phase is initiated by attacking Loremaster Angoth or stepping into his room. Sniping him from a distance will get the party teleported into his room. The door shuts and the Loremaster will emote:
Loremaster Angoth: "Who dares to disturb me while reading these important documents? An offence punishable by DEATH!"
He will then activate the Fire Gully Defence System and cast an invulnerable sphere around
him. He will fly up to the ceiling and emote again.
"Meet my Fire Gully Defence System. Good luck surviving that."
With his words the gullies will erupt, engulfing the upper and lower section in flames.

Gully Fire: Each turn two of the three sections will be engulfed by flames, dealing 5d6 fire damage to everyone standing in that section. If a player standing 5 feet away from a safe section he may try a Reflex save (DC 19) to move 5 feet in that safe section and half damage. If a player is standing further away there is no save.
The fires will alternate between sections in a vertical direction. So first round the upper and lower section are covered, second round the lower and middle, third the upper and middle, etc.

Levers: Indicated by the "L" on the map, one of the levers will shut down the fires and start Phase 2. Every lever except for the correct one is trapped with a lightning bolt spell (Search DC 23, Disable Device DC 18) and deals 5d6 electricity damage (Reflex save DC 17 half). Increasing the number of levers will obviously increase the difficulty of the challenge.

Loremaster Angoth: That he is invulnerable to any damage in his flying sphere doesn't mean that he can't use all of his spells on the party does it? The Loremaster will especially use spells that slow his enemies down so that they will perish in the flames before reaching the levers.

As said, activating the right lever will shut down the flames and start phase 2.

Phase 2:
The fires die down and the door to the library opens.
Computer-like voice: "Fire Gully Defence System shut-down completed. Thank you for using our Dwarf Defence Systems product."
Loremaster Angoth: "Damn you marauders! Stay the hell away from my spells and books."

The trick of this phase is that the party must find a scroll in the library that can dispel his invulnerable sphere.

Bookshelves: The library bookshelves can be divided in six sections. One section contains the scroll. A player can search one section per round making a DC 20 search check. A successful check in the correct section yields a Scroll of Greater Dispelling that any caster class can cast. If no caster is within the party, a party member with Use Magic Device can make a DC 25 check to cast it.

Loremaster Angoth: The man will follow the party in his sphere and still cast spells on the party, but will generally refrain from area spells that will destroy his precious books.

When the spell is cast on the Sphere phase 3 starts.

Phase 3:
Loremaster Angoth's bubble pops and he softly feather falls to the ground. Then he emotes.
Loremaster Angoth: "Argh! My sphere! It doesn't matter though. I'll kick your asses on the ground."

Loremaster Angoth: Without any advanced protection he should be like a regular caster. A good choice of spells would be crowd controlling spells like Grease and Deep Slumber.

After his death Door 2 opens again and the party may loot his helpless corpse and torch his library like good RPG savages.
Death emote: "Be good to my books..."