Although she is quite weak, comparative to the other Demon Masters, I would not underestimate Bael. 

She has spent hundreds of years refining and trying to understand how Demonic and Human mind's work, on top of having access to potent stores of Demonic Energy.

Whereas a demon like Vapula could smash an opponent into paste, it is more likely that Bael would get the opponent to do her work for her, or in some way compromise their ability to operate.

It is only the fact that the Demonic Masters guard their minds and resources so jealously that she is kept in check.

-Notes on the Demonic Masters, by Dzu