Distant Counter Spell (120 feet) or Subtle Counterspell.
So if someone teleport (Chance to fail) and try to use True, Polymorph or anything he obviously goes counterspell.
Greater Invisibility is useful for daylight and out of combat.
Darkness for the night and combat.
Darkness (Area effect) or Twin (The villain and Simulacrum) Invisibility = Simulacrum's Concentration
+ Pass without trace (Area effect) = Villain's concentration.
If you use enemies abound or any concentration, cancel Greater Invisibility, but pass without trace continues.
He will only counterspell if the enemy if he is in counterspell range, while the enemy are out of reach, he continues blasting them (1200') and sending his undead to attack them.
Right, so at 1200', he blasts, because the party can't be invisible or use cover or anything. The villein and his simulacrum are ineffective at that range -- a single round of sniper fire will find the party retreating out of effective range to regroup or just taking cover and enacting their counterplan. Range will rarely be a serious factor outside of a white room blank plain.
And the undead -- we're talking animate dead, right? So 5 skeletons or zombies per casting? How many casts? To get to numbers that are more than minorly distracting to a 16th level party, that's going to seriously cut into available spells -- in fact, maintaining, say 60 of these low CR creatures, which begins to become a threat to the party, requires 12 castings of a 5th level spell
daily. That's a lot of resting time needed just to maintain a group of undead that incinerate in the first fireball or turn attempt. We should pretty much ignore the undead and even a credible threat.
So, then, let's address the sneaking up and dropping enemies abound. The plan is for the villain, under greater invisibility from the simulacrum, to sneak in maintaining pass without trace (I'm away from my XGtE, does this show up on the Divine Soul's list?). Okay, sure. They then cast subtly Enemies Abound at a range of 120'. You're already using subtle spell, so you can use distant spell as well due to metamagic stacking limitations. So any party knowing they're up against an uber-caster with access to invisibility is running true sight or see invisible -- and they see the villain, 120' away, casting a spell. A bad turn for the villain ensues before they can escape via teleportation. Also, no component bag stealing, so sorry.
But, let's say the party gatecrashes the villain by teleporting within range for an alpha strike. They come under fire at range, hunker down, maybe with a wall spell, prep, and then pop over. They time it via readied actions so the teleport gives everyone at least a strike on arrival. That's 5 best goes from whatever classes you muster. If I'm really taking on this caster with the best I can put together, that's 6 full caster classes. You can counter spell twice, maybe, as you'll have to hope that you guessed the right spell level being cast and make the arcana check, which will be at +5 max anyway. So a few 8th level spells and they'll get through with a few. If the villain survives, it can teleport away, maybe.
Finally, let's address the failure of a simulacrum that was created. It cannot coffeelock because simulacrums cannot regain spell slots. "But it's not regaining spell slots, it's burning warlock spells every short rest for sorcery points, which is can do by the rules," you say. To which I ask -- how are you recovering those warlock spell slots on a short rest, Mr. Simulacrum? So, the simulacrum is moderately useful, but cannot burn spell slots or even sorcery points with effective abandon. It can be burnt out in short order. It certainly won't be maintaining the army of undead, or able to command them (only the caster can do so).