The witch in Act 1 and Mordus also have some 100%s (fire and earth) so yeah it just keeps happening that one of your elements don't work and ofc you could just switch your element before every fight with such an enemy but since that requires meta knowledge it is really annoying and having to respec everytime is not fun either.
Agreed. This is what sadly makes phys strictly better than elemental damage, since phys doesn't have all that added fluff to deal with.