I think you are misunderstanding the challenge of this situation.
1. He obtains a 1200ft range and can knock back on hit.
2. He flys.
3. He uses a simulacrum (allowing 2 concentration spells to be used at the same time).
4. He obtains a very high stealth score and uses invisibility a lot.
5. Most spells don't break his invisibility to cast as he can use subtle spell.
Assuming there are no errors in being able to do these tricks (I think there are but that's a different issue...) But assuming there are no errors in being able to use these tricks as described that makes a very hard to approach enemy.
How do you approach something that's 1200ft in the air, invisible and more stealthy than your rogue that you can track you without you knowing it's there so that he can pick an opportune time to ambush you?
There's a magic rune in SKT that doubles your ranged weapon attack. It's also available for a 2 level dip in Rune Scribe (and this is why he added the stipulation "No UA material," to counter my build). This makes your longbow have 1200' range and no disadvantage for range. Oathbow gives you advantage against the target of your oath.
Three level dip in something that grants see invisibility at level 2 counters the invisibility issue.
17 level champion has a crit threat range of 18-20. With advantage, super advantage from elven accuracy, and lucky, you'd have a 48% chance to crit. Now just maximize your crit dice. I had a build that was doing an average of 100 damage per shot and 8 shots per round for the first two rounds. That build was why he added immunity to poison to the coffeelock.
But range doesn't really matter, because the coffeelock is also using counterspell to counter any spells of 8th level or lower using his 8th level slot. This puts him at a maximum range of 30 feet. Did you notice that? He's at a range of 1200 feet using spells with a range of 30 feet, thinking they're a viable tactic.
Plus, he claims that dispel magic is an attack, and therefore won't bypass sanctuary, which is flatly wrong. He doesn't even know what an Attack is according to the rules of the book. He makes up his own rules.
Also note that he's not hidden unless he actually takes the hide action. And if he's in darkness, well, you can actually see a giant globe of darkness floating around, meaning you know where he is.
His tactics involve using multiple concentration spells, so as soon as he uses one, all others cancel out (including greater invisibility), meaning that he's now a target in the air with no cover.
His undead "army" is a maximum of 8 undead (or up to 16 for a single 24 hour time limit). The 16 costs his simulacrum it's 5th level slot. He also doesn't get to choose to have low level undead mages in his army with Animate Dead; it's skeletons or zombies only.
His use of Major Image multiple times takes up his higher levels slots. We already know his 8th is gone from counterspell, his 9th is gone from wish, and his 6th is gone from contingency (unless he took a long rest in the past ten days, removing all his unlimited spell slots of 5th and lower). That gives him his 7th level slot. Alternatively, perhaps he used up all his higher levels slots making an undead army or major illusions - that allows you to cast a high level dispel magic that he can't counter, removing *all* his illusions, invisibility, defensive buffs, etc... Or an antimagic field around him, destroying it all.
That's the glaring problem with this character. He uses way too many actions at once, ignores spell rules, uses too many higher level spells without a long rest or has consumed all his sorc points, and much much more. This is a Schrödinger build. (I'll also note that his build no longer has resistance to fire damage, so meteor swarm is now an effective tactic again; it'll kill all his undead and his simulacrum, and kill him on a failed save).
