Inspired by the answer to this question about sunbeams, I realized that there may be a way to exploit Simulacrum to do basically anything an infinite number of times.
Specifically, the course of action would be this:
Be Xanar, a 17th sorcerer. At level 17 you chose Simulacrum as your new spell known, and used your per-level replacement to replace one of your previous spells with Wish. You also know Dispel Magic, and took the Magic Initiate feat to pick up Eldritch Blast.
Cast Simulacrum on yourself, creating a simulacrum with all of your spell slots except the 7th level one. Give it a forked, metal rod worth at least 250 gp, attuned to the Abyss.
Give your simulacrum the following command: "Cast Wish to create a simulacrum of the wizard Xanar. Then immediately repeat this entire command to the new simulacrum. After doing that, take the forked rod and us it to Plane Shift to a random location within the Abyss, dropping the forked rod before you leave. Then search for the nearest demon if there is one, and cast Eldritch Blast on it. Then, cast Dispel Magic on yourself, intentionally failing the saving throw."
Your simulacrum (henceforth Simulacrum A), acting on your turn, casts Wish using it's 9th-level spell slot, creating another simulacrum of Xanar, who still has a 9th-level spell slot. This new simulacrum (henceforth simulacrum B), will thus also still have a 9th-level spell slot.
Note: We aren't using Wish to cast Simulacrum using the "replicate a spell" feature, since that would require being in range of Xanar to cast it. Instead we use the second option to wish for the simulacrum to be made no matter how far away he is. This isn't asking for much beyond the basic, and so should be a valid wish. This does incur the 33% chance to not be able to cast Wish ever again, but that's for your simulacrums and thus doesn't matter.
Simulacrum A, following your order, repeats said order to simulacrum B as a free action.
Simulacrum B, following the order of simulacrum A, and still acting on your turn, becomes the new simulacrum A and repeats steps 4-6.
At this point, since all newly created simulacrum act on the same turn, and all cast Wish on that turn, an infinite number of simulacra are created. On the next turn, those infinite simulacra continue to follow the rest of the order they were given, leading to:
The first simulacrum uses the rod to cast Plane Shift, dropping the rod just before it leaves. The next simulacrum then takes it's turn, picking up the rod (interaction), casting Plane Shift (action), and then dropping the rod as well before it leaves (free action). In this way, the rod travels along the infinite line of simulacra, allowing all of them to cast Plane Shift.
Infinite simulacra of Xanar appear at every point in the Abyss, and cast an infinite number of Eldritch Blasts upon every demon there.
Every demon takes an infinite amount of force damage, and dies.
Every simulacra casts Dispel Magic on itself, and ceases to exist.
The important thing here is the difference between "arbitrarily large" and "infinite". Most supposedly "infinite" tricks in DnD are really just arbitrarily large, which means that they can be repeated any number of times, perhaps even over a very short period of time, but ultimately they need to stop at some point. That number can be as big as you want, but it can't be infinity.
This is important since the Abyss is both infinitely large and contains an infinite number of demons. Any spell which can simply kill an arbitrarily large number of demons would be insufficient, since no matter how many you kill, there would always be an infinite amount left.
Since this trick is recursive, however, and happens on one turn, it actually is infinite, and can therefore be used to kill all infinity demons. Since Eldritch Blast always hits on a 20, and no demon (that I could find) is immune to force damage, and there are potentially infinite Xanars ready to cast it on them, it doesn't matter how hard any given demon is to hit or how many hit points they have or whatever.
Note that this all does assume that a wished-for simulacrum can take an action immediately, on the same turn that it was created. Otherwise, it would take an infinite amount of time to create the infinite simulacra, which would defeat the whole purpose. For the purposes of this discussion, we will assume that this holds true, though a DM could obviously rule otherwise.
Aside from that issue, do you guys see any flaws in this plan? Any improvements that could be made?