Magic missiles solve the problem of hag's illusions (and I have 2 casters, gale and astarion as arcane trickster), and astarion is fast enough to save Mayrina in first turn. Shadowheart was only casting Guiding Bolts, and my ranger (I took battle master's skill at 4th lvl) frightened Ethel, so the second turn was the last one for her.
Although I do think because BG3 is lacking the Shield spell, magic missiles are much more powerful than they should be all the time.
And fear shouldn’t provoke fleeing and cause an opportunity attack. In fact frightened creatures seem to not do anything other than run. Not that this is the reason you killed her but just something I noticed in gameplay.
Magic Missile is amazing in the current meta. It's the go-to spell for so many reasons. Part of the reason I'm unhappy with the current combat meta is wizard doesn't feel like a wizard. It's a machine that only uses jump, magic missile, and shatter.
A lot of fights after the goblin camp have your party starting on lower ground than your enemies (the Duegar fight, the gnolls), the Githyanki patrol misty stepping to the high ground, Gale in darkness (why use a turn to light up the Underdark?), dancing lights can block projectiles, threatened providing disadvantage while Gale is being chased by minotaurs, etc.