Spellcasters are out of whack. Flameskulls are cr 4 can use fireball , CR 12 npcs are level 18 casters.They usually don't have good spell lists but meteor swarms are for level 10+pcs and raw you can encounter CR+7 or so ss a deadly encounter.
With 5E saves being meh npc spellcasters are often a lot bore dangerous than brute types.
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