Is that possible? I used the Infinity Explorer to check its stats, which revealed that it had a 100% resistance against crushing and slashing damage. However, its remaining resistances (e.g. fire) were all at zero percent. Still, attacking it with the Brimstone Hammer - a weapon which deals fire damage - seems to conflict no harm to it.
If anyone knows a strategy to dispose the spirit, if it now isn't invulnerable, then please share.
Isn't that for the player to decide? Actually, if it had been doable, the most rewarding way to handle the Mausoleum area, would have been to finish the spirit's quest and thereafter kill it.