The holy water thing sounds kind of interesting, but only provided that you had one of the liches carrying a book of some sort that filled you in on the fact that you had to do that.
You could include a copy of one of Elminster's Ecologies dealing with demiliches.
In general, I've always felt these were one of those creatures to be encountered rarely because they're so damn
scary. You respect them for their power or you die. Anything that helps get that point across I'm all for.