[@VitaVitaAR]Beyond Medea's skillset as Caster? You flatter me, but no. These are curses that rapidly accelerate the decay of flesh, based in the concept of a memento mori or inducement of illness taken to its logical extreme through rapid necrosis, not giant death lasers that eradicate all in their path. As for moving the attack to point A to point B when it's launched, although it's seemingly like teleportation, that's actually just a usage of Flash Air, swapping one thing for an equivalent. Since both the curse and her familiars' down takes the form of crow feathers, the swap was made possible, although the difference in Prana intensity makes it inefficient. Flash Air is also an imperfect transposition which degrades the things swapped, reducing the curses' power somewhat as a drawback, and necessitating the absolutely massive expenditure of Prana she just used in order to overclock her curses to the point where they'd still be effective even after being replaced.

As for picking out her target, that's why she didn't go for Assassin. The Mist covers a dozen to two dozen meters, if I'm not mistaken, but Elizabeth was taking off running with Reinforcement magecraft in the direction of the Church - an easy target for Mira's crows to spot. Speaking of which, the reason she's less efficient outside her mansion is because in order to perform Flash Air, she needs her familiars to spot for her... all of which are currently located right here because she was gathering them to her location throughout the Rider fight to keep an eye out for ambushes. Even so, it's still less efficient, which is why she utterly burned herself out launching one attack.

TL;DR: This is pretty much the equivalent of Tohsaka throwing out a Jewel storing high powered magecraft at Saber during the prologue - a desperate attack with as much power as the caster can muster, but lacking in much precision or guidance after being fired. The curses don't home in on their target, and can still be blocked with magecraft - or by mundane means, since if they don't come in close proximity to flesh, they can't apply their effect - or neutralized even after they hit through the expenditure of Prana. Also, she literally just exhausted herself with just one spell, so this isn't an accurate measure of Mira's average single target offensive power, but is closer to an estimate of what her most powerful direct attack would be. Like I said, she's completely panicking and just really desperately wants everything threatening her to either go away or die, so she's basically just throwing out everything she has and hoping it works. Because of that panic, though, it's unreasonable to assume that Mira won't take a parting shot at Elizabeth, since she's the only enemy that she can see and there's an unseen murderer ready to stab her in the back now that her Servant is wounded. If it does become a fight and she sees how badly wounded Saber is, though, she's fully prepared to just use a command seal to nope them both back to her mansion, registration for the war be damned.

EDIT: Like, normally I'd just remove it, but I can't see somebody as panicky as Mira who always tries to psyche herself up by telling herself she can take on any enemy just rolling over and saying "Welp, guess I'd better just let this Servant keep stabbing me." It was either that or just firing off curses all around her - which doesn't really make much sense when she can't see her enemy herself, and a Servant could much more easily dodge - or burning a Command Seal immediately, in which case she'd probably order Saber to kill them rather than fleeing because holy shit they nearly just murdered us we've gotta stop them.