[Morgeruat]

Aye, Helm killed Mystra because she was stripped down to avatar power, while Helm still bore the power of a greater god (I believe he was still a greater god at the time).

Lolth last time I checked, was a lesser god, with the other drow gods (other than the "elder elemental god") being of equal rank or lower.

That said, it still primarily depends on where the fight is taking place. if it's in the demoweb pits, Lolth will win, in Mystra's home, it's all Mystra, On the Prime Material Plane, say Faerun, out of doors, away from their temples and followers, I would give it to Mystra, as she is greater in power and was a mortal (and thus knows all about PMS and can tap into that rage, and turn Lloth/Lolth into a chocolate bunny to appease her PMS rage).