What type of damage do undead mobs do?
Skeleton mobs:
Archers - pierce and physical; “deathbolt” types do vitality damage also.
Knights - physical and bleed; they also reflect damage back (% depends on difficulty);
Warlocks - vitality and chaos;
Revenants - fire, cold and lightning (depends on type).
Skeletal monstrosities - massive amounts of physical damage and internal trauma.
Ghost mobs:
All types - pierce, cold, physical (in that order);
Unique monsters and bosses:
Grand Priest Zarthuzellan - fire/ burn (I think this is actually obvious);
Illgorr, the Eternal - physical. 2nd form does also enormous amounts of bleed damage.
Alkamos, Lord Executioner - cold, pierce, physical (in that order); can also spawn with corrosive weapons (+acid/poison).
Moosilauke, the ChillWind - physical (>6k on ultimate, probably even higher), cold; It also freezes you for 1 second even if you have full freeze resistance.