Yes, in the upper levels of the temple of Ramm there are two types of monsters that I dread (though they are very rewarding - item- and EXP-wise):Priests of Ramm and Mindflayer. They usually appear in large groups and cast multiple Death Wishs, Instant deaths, mindflays, silence etc. at my group. Despite the magic screen being up, they often kill one or more chars --> reload. Because they are so many and have plenty of hitpoints, it is difficult to take them out within one or two rounds, so that they can devastate my group for several rounds. Furthermore, they have a high initiative and a high magic resistance, which makes it so much more difficult to quickly despatch them.
And yes, Twysted sylphs are awful to fight, they must have an AC of -30 or something like that...
Thinking of it, there are several monsters, that are extremely difficult to kill:
The grandfathers (Ninja-type-monster): plenty of attacks, poison & instant kill...
Earlier in the game, the Men'o'war usually wipe out my entire party, before I can even scratch them...