The spell "spectral stag" from player's guide to faerun makes a pseudo-creature, of the unseen servant, mordenkainen's hound, etc variety that has like half the stats a creature needs.

Of particular note is that it has a handful of hit points, and when they are depleted, it disappears. However, it also can be neither attacked nor damaged.

Obviously, the actual explanation for this is the lack of editing and overall laziness of the devs, but out of idle curiosity, what are all the ways the spectral stag specifically could use hit points without being attacked or damaged?

It's not a creature so can't take class levels and manifest empathic transfer or anything. If it walked into a greater consumptive aura, would it lose hp? Aoes just damage normally so those are probably out.

Is there a way to deplete a spectral stag's hp and make it disappear, or not?