Do you think the Retriever will kill the Wizard alone? You will be on fire all the time. You will choose to fail at Paralyzing Beam. Well, it's cool. Fail every dex saving throw. It's Instant Death.
Of course he'll have to try to retrieve (not kill, it's not what retrievers do) the Wizard 20 (let's call him Alfred the Homebird, 'cause he will defeat the HUN-HAT from the comfort of his own home (a rural cottage in a bucolic village not from the city where Binky lives) once all the details of your setup have been clarified. The retriever will have to go alone because (a) he knows where to find Alfred but can't communicate the information to anyone, lacking a language, (b) he will planeshift to a location and his variant of planeshift only makes him able to move himself and an incapacited person.). The HUN-HAT can't planeshift. So if he tries to ask the Retriever to take him to Alfred, the net effect will be that the HUN-HAT will have to be incapacitated, and delivered straight to Alfred. Now that enough fact about the HUN-HAT behaviour have established, we can start the duel.
Recap of prep time: HUN-HAT has spent an unspecified number of years but at least 3 befriending a drow matron mother and building impenatrable defenses which he doesn't really uses because he needs to hover all the time above 40 feet and has no way of getting to when he gets back from his Demiplane. Meanwhile, Alfred the Homebird developped gardening skills and became a recognized expert in catnips cultivars. He is somehow tasked by Ilmater to put an end to the HUN-HAT misery, and feeling playful, Ilmater didn't give any detail on the poor sod, except his extremely discomfortable life...
Day -X to day 1: following your setup, Alfred tries to Scry upon the HUN-HAT and fail, because somehow HUN-HAT was in the Dreaded Demiplane of Basements. Note that Alfred at this point was scyring in part because he wanted not to irk a God and accepted the task, but also in part because he smelt somthing fishy as usually, the Weeping God of Compassion doesn't bang on his door handing out murder assignments. So he was suspecting it might be a clever practical joke setup by Binky (who is making AWESOME voice acting since he took that Bard Initiate feat) and just wanted to check if the HUN-HAT life was as sad as "Ilmater" said. Since the Scrying failed so far, the HUN-HAT is unaware so far of the existence of Alfred and his attempts.
Day 1 : for the first time, HUN-HAT gets back to the material plane (this was part of your setup). As it is impossible to get back without using Wish on this day, HUN-HAT has expanded his 9th level slot, don't have Mind Blank, and his somewhere away from his base and defenses. Lacking any mean of quick transportation, he'll walk toward the base, while the Scrying will succeed for the first time. At this moment, SOMEHOW, he'll know that the scrying sensor belong to Alfred, who probably had an affair with the DMM in the past -- Binky wrote a book detailing their passioned, albeit short-lived, romance (a contrieved setup but as all parties agreed upon that, it's OK -- and ask the DMM to summon a retriever to go after him (established in post #312). Then, as he is intending to reach the wizard, he asks the retriever to bring him there and the retriever promptly comply, incapacitating the HUN-HAT and bringing him in this state, through two planeshifts, to Alfred's smoking lounge, as you established in post #326.
I suppose we got enough elements on HUN-HAT behaviour to start the duel from this point. I'll try to provide a build for Alfred and show how he can defeat the lone hostile retriever and the incapacitated HUN-HAT, who put himself in this situation WITHOUT ANY ACTION ON ALFRED'S PART SO FAR EXCEPT SPENDING A 5TH LEVEL SLOT TO SCRY AND NEVER LEAVING THE COMFORT OF HIS OWN HOME. Really, I must give you credit: the world-conquering plans of the HUN-HAT are really INT 10-like, you're roleplaying him well.
[As this exercice is still fun and I am sure other opportunity will arise, I will allow you to retract the fact established on post #326 about the HUN-HAT going with the retriever... I'd prefer Alfred to win without having to roll initiative, as he is quite minding of his comfort and dislike the stress. That's why he left the big city.]
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