Adjust.
Seriously, if an organised party keeps encountering a character "whom weapons cannot touch" (assuming they leave survivors), the that party is going to switch to something other than weapons. Add more spellcasters, more spell scrolls, attack more than just their AC.
Seriously, if an organised party keeps encountering a character "whom weapons cannot touch" (assuming they leave survivors), the that party is going to switch to something other than weapons. Add more spellcasters, more spell scrolls, attack more than just their AC.