"The Will of Ashan" - "Heart of Darkness" [2nd campaign, 5th mission]

the main thing is to start by recruiting Arantir, level up his Spell Power, and I also took the Skilled Sorcery skill... if you level it up correctly, he'll have 40+ spell power by the time you reach Holy Flame, which eliminates the need for a Death Avatar because the Phoenix turns out to be much stronger!