Proposal: Eliminate the Holy damage type.

[irrevenant]

"Ethereal" makes sense since it's damage to the ethereal body, but you're right - it's probably not a common enough concept to be intuitive.

I think Rhuvaen may have hit the nail on the head: "Spirit" or "Spiritual" damage fits the bill. Arcane attacks directly attack a creature's 'soul', 'life force' - or spirit.