Nyriss would kill him in a great fight if they were restricted only to lightsabers; she is beyond him given how stomped Meetra & Scourge were by her, but Sidious has far too much power, enough to the point of throwing her across the room or killing with Lightning.
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They are angry because they have been forced to recognize that their hour has arrived; that the time has come to surrender power to Shimrra and the new order."
Nyriss ashed herself with her own lightning, Sidious doesn't even leave burn marks on people when he blasts them with his ( see Mace/Yoda in ROTS, Luke in ROTJ ).
Palpatine has trouble with two Sith, but Meetra beats like... a million. And Nyriss stomped Meetra who had aid, so she speedblitzes Sidious in a duel.
As for Force powers, Sidious couldn't even kill Dooku(Son of Dathomir), Mace Windu(Return of the Sith) and a blonde *****(Revenge of the Jedi), whereas Meetra could've been reduced to ash by Nyriss(even though, moments prior, she survived a blast from her lightning). Nyriss would one-shot in a Force-only battle.
__________________ "Happiness is a lie. Life is horror. The light is always dying all across the universe. The last star will flicker out someday, when it does, all that remains is shadow. And I will be its king!"'-Amahl Farouk