I always took Egwene's chances in a one power duel against most of the female forsaken to be the same as her chances against Nynaeve. Egwene is strong enough to use barefire and to travel and if she was really lucky or caught the forsaken by surprise she might be able to kill them. In a straight up fight however she was really outmatched.

The one she is probably closest to is Moghedien but even against here there is probably more that a single level gap in strength. What ever the gap with Moghedien there would be a furhter gap against Mesaana with probably another gap againt against old strenght Lanfear.

Judging by the fact that the forsaken have not automatically killed off their weaker members I would guess one does not have to be exactly equal in strenght. A level drop might be still enough to make the conflict uncertain but once you start dropping down two levels you are getting a lot closer to the stronger channeler being much more certain of victory and more likely to strike. Drop down three levels and you are hoping for luck to keep you alive.