I think that Sheriam would know the potential strength of Nynaeve and Sharina. It has been remarked on as being greater than any other Aes Sedai. She is so over-awed by the strength of the woman outside her tent that I feel Mesaana must be stronger than those two.

In the Prologue to LOC, Mesaana thinks to herself that she matched well with Semirhage. So Semirhage would be stronger than Nynaeve too.

In TSR where Nynaeve fights Moghedian and later in TFOH where she captures her, I did not think that Nynaeve was stronger. I thought that she was close to Moghedian's strength but won by using her wits. Didn't she throw something at Moghedian that distracted her in the TSR fight? And putting the TAR adam on Moghedian in TFOH was pure quick thinking.

I have always thought that Moghedian was not one of the stronger Forsaken and that Nynaeve was about average for forsaken level chanellers.

Moghedian escaped in TSR with 1 channeler shieding her. Was this only because that shield was tied off? Perhaps 1-2 chanellers could hold Semirhage as long as it remained ongoing and not tied off.