Went into this fight buffed to kingdom come, particularly on the main tank, Kagain. But even with untouchable saving throws, the auto-trigger by Senaj sends a Death spell towards Kagain, and he is killed (but his icon remained, so could theoretically be revived). He was level 8, and I see in the spell description that people above level 8 are immune, so I guess that's why it happens. It also says they can't be revived though. Can't recall this spell happening in the Tutu way through the game, so is this a BGT special cheese?

When the same thing happened twice, he also got a scroll with 50% magic resistance. The coin landed on the right side and he survived, and we went on to beat all of these bastards. Short of magic resistance of one type or another, is there absolutely no way to defend against this?

Also keep in mind that we were "overpowered" due to playing via BGT, and had almost 250,000 XP each when normally the cap is 161,000.

Could have sent Imoen up to take the heat I guess (thieves level faster), but that has the possibility for all kinds of other undesirable events, given her poor health and so forth.

Off to BG2 we go, but this fight was even rougher than expected given our high XP and massive buffs.

I'm pretty sure the Finger of Death spell or whatever it's called was added by Baldur's Gate Trilogy for reasons :-P

Yes, very much so. That spell should not be available. Semaj should be firing Chaos, Domination, Cloudkill, Greater Malison, Fireball and Lightning Bolt as he dimention doors around. Angelo also uses Arrows of Detonation. Then there's the immovable traps in the center.

Some of those is why I buffed with protection against fire and electricity, and a plethora of other buffs. Bit sick when nothing helps, because you're struck with something you can't defend against, short of a lucky roll on magic resistance. It's why I hate the Aec'Letec fight too, plus that damn Kangaxx and his Imprisonment or Maze or whatever it was.

Semaj didn't dimension door around either, or very little. I remember in the Tutu fight Semaj would end up in the back of the room pretty quickly (where we enter), but here that didn't happen and he was in front of the traps. Sarevok hits hard, though, bloody hell. Always glad when that fight is over. It's good with tough fights, especially at the end of games, but I don't like it when they pull off stuff they really shouldn't. Death spell isn't even memorised, it's fired from a script (yes, I had to look up how this stuff could happen).

If you want to get revenge on the game (and aren't above cheating), here is something to try:

Start a new game and give your character the Deathblow ability. That ability is often listed as useless, but that's only the case at the levels at which you normally get the skill. In Baldur's Gate 1, on the other hand, pretty much nothing is strong enough to avoid being instant killed. For best results, give yourself a large number of uses, and use darts (3 attacks per round) as your main weapon. I believe it also works well with area effect arrows. You can even kill the guy in the Baldur's Gate 2 tutorial this way (if you don't mind softlocking the game; I don't remember if it has to be Greater Deathblow).

(Is there anything in Baldur's Gate 1 that can survive Deathblow? Anything that can survive Greater Deathblow?)

Of note, Scribe Scroll is another ability you might look into. It is likely overpowered in BG1, reasonably balanced in BG2, and underpowered in BG2's expansion. There's also Create Wand (which is actually an effect of Wish) to get ulimited wands (with 5 charges each).