Missiles I can grasp. BoIB, YDS, DoE and their natural resistance will give them 115% Resistance to Missiles. Though I don't see how they can permanently get their resistances against crushing and piercing damage up to more than 90%, which is what YDS, Roranach's Horn, DoE and natural resistances will give you.
Slashing weapons I've no clue, a hacked Barbarian/Thief with the Jansen Armor could get this, though he'd not be immune to piercing attacks in that case.
If we're counting in the Ascension bonus it would work, but what have you got left at that point in the game anyway?
