ToME: the Tales of Maj'Eyal

This is on Nightmare difficulty (generally, though I've hit this wall on Insane a few times as well)...

In one of the prides is a murderous vault with an armada of difficult enemies, all of which are easy enough to kill save one: Bethana The Caustic Terror.

I've had numerous characters tear through the prides, win the game, and a decent number took down Atamathon but none has yet killed the Caustic Terror. Any tips?

I also met with this vault (iirc at the very first time) recently, and it was a crazy challenging fight.
I could clear the Vault, but for me the Frozen terror was the craziest dude.
I guess, they have 3 random classes with the few fixed talents that they have, so the random classes can really
affect the powerlevel of these dudes.

With Fallen, it is definately clearable:

https://www.youtube.com/watch?v=WEjtR7K6_Hg

I've never had a problem with any other one, though once I had a half-hour stand-off with the fire dragon but I got him eventually.

It doesn't seem to matter what classes he draws... that SOB is murderous as hell, ha ha.

Good times.