That isn't even the worst, in BG2 if you are "immune to magic" you can not be time stopped, which means if you cast time-stop as a mage, and your enemy is immune to magic, your party is time stopped but your enemy (and mage) isn't, which of course you have absolutely NO WAY of telling before casting it once.
Why do I mention this? Because of Demogorgon, of course. I never beat the thing, mainly because despite having an absolutely well equipped party with most of the absolutely most fancy stuff, there was no way for me to enter there without INSTANTLY being pwned (if you don't cast it, he casts time-stop on you which is even worse, because your spells don't effect him during a time-stop, so you have no way to reduce his magic immunity or silence him).
The funny part is, if you are immune to time-stop, your arrows and spells are still time-stopped, so you had to melee him, which meant.. dieing in 3 hits. oh well ;P only enemy in BG2 I could never beat. Actually the only enemy in BG2 that could wiped me in 2 "ticks" because when I first entered only my wizard of doom was immune to time-stops, so that meant my wizard vs demogorgon.. was a very short fight, especially once I realized that my main-stay support spell (GATE) didn't work here, because he is the prince of demons, he literally pirates your gate and uses it himself.. bah ;p
Although, to be fair the game did tell you "You really should not open this seal, like really"