There is actually no evidence that Alivia is the strongest of all damane. It actually would be very surprising if of all damane, Rand's troops managed to capture the strongest of all Seanchan damane in PoD. Most likely some super strong damane are in the possession of the Imperial Family. It makes sense then that Tuon took at least one super strong damane with her for the campaign in the Westlands.

But maybe BS will clarify it in coming Q&A (maybe he would answer "Is Alivia the strongest damane out there or at least the strongest among those we have seen in the series?", or Sera will play a more important role in ToM and/or AMoL, which shows her true strength.