Divinity Origin Sin 2 - Alexander Boss Fight
The fight is not easy but a party of your level should be enough. Without knowing much of your playstyle during that fight, my only advice would be to play low : when the worm appears, ignore him, stay away from him, and attack the magisters (yeah, even if your said that "you have to team up against the greater ennemy"). They will mostly attack the worm so you can use that time to effectively heal your party and weaken the magisters without getting to much retribution for it.