That's the problem with Dc fanboys they think is always about strength and speed it's not, if that's all that wonder woman has to fight thena the fight will be over very quickly. Thena has more powers than wonder woman, wonder has fighting skills so does thena, eternals are like earth base cosmic beings, eternals were given powers by the celestial a weakened Zuras fought Zeus to a stand still a skyfather and thena is the daughter of Zuras and she is no push over.
When Sersi went crazy Ikaris the prime eternal sent sprite and thena knowing that thena was the only one that can match sersi powers and would have a better chance then the other eternals.
Ikaris (PE), Forgotten One, Sersi , Thena are the most powerful earth eternals. Any of these people can serve wonder woman, superman, martian manhunter on a silver platter.
Only two Eternals have ever been killed. Zuras who was killed by a Celestial. And Gilgamesh who was hit with a beam that stopped his body from taking in and manipulating cosmic energy. Her sword will cut her sure... But she will then reform and heal her body since she is just as powerful as Sersi. WW will think she has won, then is suddenly molecular transformed into a dog... Or a pig... Fight over.