Of course. I hope you too will be able to defeat HUN-HAT! I have all confidence.
Meanwhile, here is my course of action for the situation mentionned above.
Everything occurs like in the Alfred post up until the moment he casts Scrying. The locator appear, being seen as a glowing luminous ball by creatures that can see invisibility. Knowing somehow it's a threat, HUN-HAT engages his usual behaviour in that case: he asks the drow matron mother to cast her unique and non renewable Summon Retriever and order her to run toward Alfred. The HUN-HAT takes not of the bearing and starts destroying everything on his path, seemingly ready to do that for the next few months.
Alfred then had a pause, while facing his mirror revealing the surrounding of the HUN-HAT. He knew the girl. Her once-fierce look had subsided, replaced by a demeanor of subserviance, but he recognized her fine black skin and her silky white hair. It was Eilirilia... she hadn't changed over the past 40 years... Then, Alfred wept. He had thought the avatar of Ilmater knocking on his door was a prank. But no. It was not. Ilmater really spoke to him, not because he wanted the HUN-HAT dead, that's of little consequence. He wanted to see how his selfishness in his love affair led Eilirilia to a life of unhappiness. The god of compassion had wanted to give him a last chance to correct his course of action and make things right. Alfred sighed and decided to free Eilirilia from the sociopathic destroyer (since he wept for two round and swore for one, HUN-HAT was at this time destroying an anthill 160 feet from his original position.
"Simulacrum, I order you to kill this guy. I want him dead. I must learn what he did to Eilirilia but he must pay so I'll go the easy way of asking his corpse. Use every means available. Walk the Path of Extreme Prejudice".
During round 5 to 10, the Simulacrum goes to the basement and make a careful path among it chanting words of power that were triggers for empowering glyphs of warding woven into the pattern of the carpet, for situations of emergency, hence the nickname his master has devised. Rarely did he allow the use of it, but today was a special day, and he seemed moved by what he saw in the mirror. Once buffed by the glyphs, who had been created over the past years (prep time), he was the subject of Greater Invisibility, See Invisibility, Far Step, Fly, Mind Blank, Intellect Fortress, Protection from Energy, Haste, True Seeing and Protection from Cameras (the latter being a really obscure scroll Alfred had copied from a tome in Candlekeep, never really understanding its use but as a Wizard is always prepared, it was integrated into the spell glyph pattern). He was ready to perform its task an signalled his presence to Alfred, whose eyes were still locked on the mirror, with grief and pain on his face.
Alfred cast Demiplane, opening the rift between worlds and reaching for his extraplanar laboratory, a supernatural extension of his cottage's closet where his treasures were stored. Both Alfred and the simulacrum entered, while Alfred gave the last orders that would seal the HUN-HAT's fate. "You can go now", did he say, opening a Gate to the material plane again, on the precise location above the HUN-HAT groups. Then Simulacrum crossed the Gate as a move action, finding himself floating above the group. The Simulacrum's mission was to kill HUN-HAT but avoid harming Eilirilia. He wouldn't disappoint his master. He would neutralize the HUN-HAT, who was clearly distinguished from his Simulacrum by the fact that the Scrying Locator was hovering over him, a telling glowing ball of light. Forcecage does not require concentration and Alfred's simulacrum made the bet the sorceror had less capacity to planeshift or teleport than he could withstand uses of Convergent Future to make him miss his saving throws, still expanding the spell slot. As a bonus action, he far stepped 60 feet up. In effect, at most he would have to spend a single use of it, since only Demiplace could qualify as a way to exit the cage.
It was obvious the Eilirilia was a Simulacrum herself, given that she adopted the same behaviour of attacking Alfred's Simulacrum as the HUN-HAT's simulacrum. With a flying speed of 30 ft, even Extended Finger of Death or Power words wouldn't reach the Simulacrum this round. Plus, the Simulacrum can't see invisibility unless he spends some time casting it. It will be a problem the next round. The fake Eilirilia was lacking any meaningful way to see invisible creatures and could only levitate upward clumsily she wouldn't be a threat to reach an invisible attacker able to take a move action and an additional Dash action of 60 ft, plus a far step of 60 feet, and still cast a spell. Carefully staying outside of counterspelling or extended dispell magic range (240 ft, this one is nasty), plus move, at the end of his turn, Alfred's Simulacrum decides to lob a fireball at the group, hurting magens and severely injuring the Simulacrum as he had just cast See Invisibility, before finishing them off on the latter rounds (spending 3rd and 4th level slots). At some point, Eilirelia would be floating slowly, lone survivor of the split group and since the goal wasn't to hurt her, Alfred's simulacrum would use his superior manoeuverability to fly around her and Feeblemind HUN-HAT (expanding his lone 8th level spell and possibly a second convergent future to ensure success). It could harm his move speed, but speed was only useful to split the group and at this point it was no longer needed (in the extraordinary event of a successful INT save) and potentially a third to ensure the failure of the counterspelling. With a CHA and INT of 1 for the next 30 days, he won't be a threat while Alfred's simulacrum kills him with cantrip through the bar of the forcecage.
"Satisfying..." said Alfred again poring over his silvery mirror... "This might not exactly be what Ilmater wanted as outcome but hey..." It is a slow death (though unexplicably, the HUN-HAT died very quickly... how could he have so few hit points? He shoudln't have made a Simulacrum of Eilirilia without her consent. Her body, her choice.
He'd have to recreate a Simulacrum anew to replace his tired butler, but that would be a task for tomorrow. Right now, it is time to make some herbal tea: Binky will arrive soon for tea and biscuits and he might be interested to learn about the strange events of the day.
HUN-HAT has been killed by Alfred, dying of cantrip, without him bothering to leave the comfort of his own home.