Various mods by zahar0z

Also, resistance to dark magic is a must-have.

Then it's better to focus on resistance to magic in general, so that a single ice block can't inflict lethal cold damage...
And crystalline scales from Death Harbingers/Cave Lords.
Of course, the main problem is that then these abilities will make up a long, aesthetically unappealing list; if it were possible to put everything the creature can do into its description instead... But otherwise, it will be impossible to create a mechanically dangerous boss.