I researched this by reading a number of forums where people doing no-reload runs ran into this and instantly had their game end, often with no message as to why (it was the no-save Death Spell or the Petrification). I am also looking at it in Near Infinity. It appears this was done to BGT around 2010, people posting in 2012 had already run into it 2 years before.
Angelo has 120 Arrows of Detonation instead of 8. He has magic items that make him immune to spell levels 1-3 as well as normal missiles, and a ring of invisibility. He has a script that auto-casts Remove Magic, Death Spell, and Flesh to Stone at the start of the battle. He has a necklace that makes him immune to Entangle, Silence, Stinking Cloud, Web, Call Lightning, Slow, Skull Trap. on and on. He has 25 character levels. He has a Haste ring. He has resist fire 90%. I could go on.
He is basically hitting you with four Arrows of Detonation per round the entire fight (after his initial spell barrage). His Thaco is 6, he has a 19 DEX and a +2 bow, so effectively his Thaco is 1.
The View and Edit screens of Near Infinity conflict so I can't tell if he has 120 arrows of detonation and 120 arrows of dispelling, or 240 arrows of detonation. In fairness (Ha!) I am using the convenience tweak that increases quiver size to 120, so it is possible he normally comes with "only" 40 arrows of detonation.
This is just Angelo. They all (except Sarevok) start invisible and they all have a pile of special powers and immunities not in the original game. I did this fight three or four times and never saw an enemy fail a saving throw (I was using two mages, a cleric, and a thief and two fighters who all used ammo that required saving throws with each hit).
It's definitely BGT doing it. Dudleyville gives you the original game item list
http://www.forgottenwars.com/bg1/ar0125.htm
Looks like Sarevok has 46 levels unless I am reading something wrong, all saves are 1, Thaco is 2, 285 hit points, 90% resistance to all non-melee damage except regular missiles is 50%, 4 attacks a round with ring of haste. Sarevok's effective Thaco with his strength and sword is -6.
Apparently, Arrows of Detonation have the same effect as a fireball, but don't count as a "spell," so Minor Globes and Protection from Magic scrolls do not protect against them. Only Fire Resistance can protect you, and of course Angelo's opening move includes Remove Magic.
So they are not breaking any rules here, other than basically making the enemies immune to anything you can do.
Except Ctrl-Y of course.
At least this means BG2 probably works, unless they decided to "juice up" some of their favorite encounters there as well. This doesn't appear to be a case of something not working, so much as ridiculous encounter design coupled with the enemies essentially being extremely advanced BG2 high level enemies (with unique artifacts) against your BG1 level party.
The old tricks of pulling individual henchmen (they are invisible), having tons of summons (you can only have 5 at a time, and the arrows of detonation wipe the slate) don't work. Maybe it is possible to use a detect invisibility type spell to see one of them without activating Sarevok, I don't know. It has very much become a cheese encounter.