Sadly, it's a known game engine bug (see section 5 of the readme: Valen can be "killed" by intelligence drain (and some other methods) and she won't "die the final death". Game engine bug.). I assert that much of the time she will actually be "killed" by them some other way and actually die :-), but beyond that I can't do much.
If won this battle twice, and twice they killed Valen with a Sunray or something while she was in gaseous form, and she would die normally and still be in your party. This removes all the challenge from that encounter, since you can just let em kill Valen and walk off.