Ruined City Headmaster

From HGWiki

To open the Ruined City Academy, this must be done:

Spoiler Warning!
Cast the following spells on the door/gate: On the door in the Drowned City cast: cone of cold (5) On the gate in the Academy cast: Gust of Wind (3) Chain of Lighting (6) Ice Storm (4) Mestil's Acid Breath (3) Bigby's Forceful Hand (6) ….in that order

The boss drops:

Spoiler Warning!
3 Tomes: Spell Penetration, Conjuration, Evocation Book of Races: Thri-kreen 2 Random rare items

The Academy is populated with Maze Guardians, Undead Arcanists, Balors (as per the Balors on the Immortal run) and the Headmaster himself.
Sunburst can be very effective, try to avoid killing the Balors close to the Headmaster as the ondeath kickback damage can cause the tag to fail, although you can safely kill him with Great Thunderclap as he takes Sonic very well whereas the Balors take practically nothing from this spell.