By the OP's stipulations, the 60 galvan magen are instructed to cast static discharge, for 1,300 damage, at any creature they detect through their constant search actions and that is not "one of us". A wise wording since the magen are absolutely loyal, but lack any mean of recognizing the sorcerer once he is disguised through clever illusions to look like a magen.
1. Using divine intervention (Cleric 20) applying rule as written and not rule as intended.
By RAW, divine intervention grants the "effects" of a divine spell or a clerical domain spell. It doesn't replicate the casting of the spell, it just states that the "The DM chooses the nature of the intervention; the effect of any cleric spell or cleric domain spell would be appropriate." Therefore, from the comfort of his own home, the cleric 20 could just pray for the sorcerer to Seem to be a black bear. It's just replicating the "effect" of a spell and not the casting of the spell, so by RAW, there is no need to identify precisely the target or any need for it to be in range. Seeming allows for a CHA saving throw, but unfortunately, the chances of the sorceror to succed at it are low. So suddenly, 60 magen will notice a black bear standing among them... and promptly obey and inflict 1,300 damage to the sorceror.
2. Using wish (Wizard 20) applying rule as written and not rule as intended
The cleric's neighbour is a wizard. Not wanting to be outdone by his pious friend, he casts Wish to "duplicate any other spell of 8th level or lower. You don’t need to meet any requirements in that spell, including costly components." Seeming is a 5th level spell, so it's possible to duplicate it by RAW. Of course, the wizard will cast it from the comfort of his own home, so he will be out of range and he will not be able to identify his target precisely.... But by RAW he doesn't have to meet any requirement of the spell, which include the requirement that "any number of creatures that you can see within range." So the sorceror just takes the form of a black bear.
OK, I admit that these use of the rules as written are extreme and that RAI is superior to rule as written. But if the sorceror's simulacrum are regaing sorcery points daily, the DM of the setup is already using RAW and not RAI, because sorcery points have been disallowed for simulacra by Jeremy Crawford's answer: the intend of the rule is to make simulacra unable to regain any expandable feature such as action surges or sorcery point definitely lost by the simulacrum :
If the DM of the original setup leans more toward rules as intended, scenarios 1 and 2 can't happen but the Simulacrum doesn't regain SP either, which leads to the next scenario, more serious (once the idea that simulacra regain SP is off the table).
3. Wizard 20, with a little prep work
OK, using RAI, not abusing Wish by expecting it follow RAW strictly. During the ample preparation time protagonist of the setup seem to have, the wizard has prepared the usual countermeasures (a clone, with spellbook, stored into a demiplane), rested and created a simulacrum with all spell-slot including Wish, only lacking the 7th level spell slot used to cast Simulacrum.
Day One. The Magen can't possibly identify whom among the Seeming Magen is their master. They have no supernatural ability to do so, and if they can see through the disguise, so can a lot of people given the magen very average stats. If only one of them turns into a black bear, well... they might be puzzled. But what if... all of them turn into a black bear? Or an ape? Or Aunt Polly? Their order will be to turn against each other (including, but not limited to the sorcerer) in their attempt to static discharge at the threats suddenly appearing. So, while the sorceror is outside his fortress (we are allowed by the setup to choose that, aren't we?) the wizard cast Greater invisibility on the simulacrum (using the wizard's concentration), the simulacrum cast Fly on itself (using his concentration), teleport (cast on his 8th level slot), after scrying the location outside the sorcerer's might fortress if needed, fly over he magen and cast seeming (no concentration) to make all magen-like creature appear to be Aunt Polly, causing static discharges to hit. Of course, the magens have lightning immunity, so they can't hurt each other with static discharge. The only one that will be hurt are the sorcerer, his simulacrum and his DMM. So, the invisible, flying simulacrum can just take actions to identify who is who by focussing on the bears that appears to be wounded by the lighnings and not the Pollys that seem to be doing fine. Of course, if the sorcerer just use an action to issue a new order to the Magen, like "stop this silliness", he will have exposed himself as (a) Magen don't speak (b) only their master can give them orders so it's not the DMM. Actually, with 10 INT, it is very probable that the sorcerer will make the mistake of... not casting static discharge himself at the nearest bear (provided he has a spell that can look like it, which is nothing the rules tell us) because Seeming won't change the way a spell looks like. If he is the only one (or one of the few) who does things differently than the other, he has just given away his disguise in the heat of the moment.
The wizard simulacrum would them cast (after some rounds) a meteor swarm. The OP countered in the last post that "meteor swarm hit the ground" and that "flying creatures are immune to meteor swarm". While the former is true, there is nothing in the rule that states that flying creatures are immune to meteor swarms : "
Blazing orbs of fire plummet to the ground at four different points you can see within range. Each creature in a 40-foot-radius sphere centered on each point you choose must make a Dexterity saving throw. The sphere spreads around corners. A creature takes 20d6 fire damage and 20d6 bludgeoning damage on a failed save, or half as much damage on a successful one. A creature in the area of more than one fiery burst is affected only once.
The spell damages objects in the area and ignites flammable objects that aren’t being worn or carried."
The spheres are centered on the ground, but spheres are spherical and burn hovering creatures. The only way to make sure the sorceror and his retinue are immune to it would be to never be at less than 20 feet from the ground. Which is impossible because the sorceror is using Seeming to appear like a magen, so he is not using his otherworldly wings, because he wouldn't be able to appear like a magen with them (too many limbs). And it would be to easy to spot the sorcerer if he was "the only magen that is not flying far above the ground like the other" or "the magen flying with all the other magens, except with two spectral wings". In the OP's setup, it is said that "all the opponent sees are magen", so we can conclude that the magen are just walking on their feet in order for the disguise to be even remotely useful. If the disguise is this weak, of course the strategy would be different, so let's go with a useful disguise and walking magen, among which hides an unwinged sorceror (who can sprout wings anytime as a bonus action). So basically, yes, the magen army is off. The simulacrum can die, the wizard still has 12 hours in his day to make another one as he never left his house at this point, should the sorcerer somehow, at his round, kill the simulacrum (still invisible and flying).
After this initial engagement, the magen army is dead, the sorcerer is heavily wounded by the static discharge and the meteor swarm, and he's probably perfectly identified if he tried to give orders to the magens to stop fighting, or he sprouted his wings for some reason. If he just forgot to cast static discharge, he can still be mistaken for his simulacrum and DMM, which is less useful since most magen are probably wiped out at this point.
