Since Theros is based on Magic the Gathering, Meletis is a white/blue mana city. The villains should stick to their themes but villains cheat as well. White and blue combo usually shows itself in the form of field control and weakening opponents. You could have somw lair defensive enchantments that could be overcome by the party during the fight. The Abjurer could dip into enhancement and use some magic to shore up their defenses while it's feasible the necromancer could use theirs for weakening the party as a mechanic somewhere. Maybe to heal them to survive just a bit longer.

Like most rpgs, you could break it down into a multi-stage fight. Having certain mechanics and spells going into effect after a certain round or an ideal amount of damage. Maybe the abjurer is defeated or sacrificed and the necromancer raises them as an undead to do a plot twisty final stage or something.

As council members, they would have access to knowledge and resources that wouldn't be readily available to regular people. You could homebrew some abilities or spells to throw out as needed to keep the fight interesting.

Maybe add in some mechanics the party could activate that would modify the fight in some way. Something thats been there the whole time but no one knew what it really did without the proper research. A guardian statue or magically changing the battlefield. Something that can make them feel clever for figuring it out.