It has effectively 160 hp in most cases. Either because of resistances or through its "avoidance" ability. Even so, that's not very much for a creature encountered solo at level 13-15.
Note that it is powerless against damaging spells that does not afford a save.
If I'm not blind nothing truly dangerous happens unless they touch Acerak.
What prevents the heroes from gearing up from the vault's treasures itself? That even includes all the gear stolen by the teleport traps!
Anyhoo - even if I
am blind, there is an excellent chance my players would set the Tomb on hold while they adventure elsewhere to find new magical weapons before returning to the Tomb.
If you rule its Howl and Drain aren't magical, sure.
Just by reading the MM entry, however, I get the feeling the designer intended this to be cast on an enemy. Of course, your idea is much better. Perhaps place a couple of mirrors large and small throughout the vault (it can then see the creature that is itself...

)