Her killing that general was the only impressive thing among that list.
Cutting hundreds of troops is as impressive.
If she was a man her plan would've failed ages ago.
Why? What does her being a woman have to do with her plan being successful or not? If she were a man, would you have written the story in a way they failed?
Has she not been useless and helpless always need saving these past few chapters?
No. She already did her part, now the rest can do theirs. The same way Shin was not useless when Bi Hei and his brother were dragging him around to save him from Zhao's troops.
Useless implies that even in the capacity of doing something, they failed to do it. Like the younger archer brother that could not shoot at anyone during the raid of Retsubi.
Yo Tan Wa laid down the plan, put herself up as bait, killed a general, distracted the core army of Zhao to hunt her down which allowed a detachment of her army to capture the city. Her feats go beyond martial prowess. There was no one there that could take her on 1 v 1, that's why the author chose to put her through endless hordes of fodder. And a good author would not follow that up with her fighting Rozo afterwards. It is disjointed from what transpired.
If anything, the one that should be dead is Baijo, but if anyone is having plot armor, it is him.