I think the one really tough monster in this quest should be hittable with +1 weapons. I remember that used to be the case - I've done this quest a lot in the old days - but now, a certain mob is pretty much impossible to harm unless you bring in the greataxe squad.
(Maybe I am missing something.)
This is working as intended and as far as I know they have always been like this. There are many ways to deal with the monsters.