FotJ Luke Skywalker vs. Darth Nox

Darth Nox can kill many soldiers, can Luke compare?

How does Luke stop him if he strikes high and then low?

Noxs ghosts make her functionally immortal, Luke can't beat what he cant kill

Can Luke counter her attacks?

Luke out-Marty Stu's here. Once her Ghosts come up, Luke'll pull off some exorcising power and then split her in half with his saber.

But what if she blocks him?

She can't block the power of a part-time Gary Stu.

Are you saying Luke is overpowered?

At times, certainly. But him being a Skywalker, I wouldn't complain about it.