If your character is a Warrior, again with bosses such as Josephina and Moriendor especially, I suggest a frost weapon and using the warrior's special move while invisible. I look for frost weapons constantly during game play, replacing a weaker frost weapon with a stronger frost weapon having a frost weapon in my inventory always. (see attached image) The axes have a longer recoup time so you get less strikes but with frost even they will wear down a boss. I look for a sword with frost and a low recoup time but there are others like frost axes etc. I don't want to say frost weapons are broken but they seem to be the exception when fighting the bosses.
However, a weapon with frost will freeze both, in my experience, as a weapon with frost seems to ignore any resistances from any character. Any critter with high spiritual resistance will also be less affected by the frost spell when you cast it. There are some of the boss characters, Josephina and Moriendor especially, that resist the freeze spell.