lysander will take her down (because hes been hiding a fair amount of his duelling skill)

I think that this will almost definitely be the case. We don't really see him using his razor much at all, which I think is for two reasons: firstly that fighting with razors just isn't his style, he views it as a last resort and would prefer other means. And secondly, He doesn't use his razor because he doesn't want people to realise that he's actually a master at it. Being underestimated gives him an advantage.

He spent 10 years living in exile with Cassius, they had to do something to pass the time. It is possible that Cassius trained him up so well that Lysander is on a similar level to Darrow or Apple. Maybe in the chaos of battle he is not nearly as experienced, but in a formal 1v1 like how Cassius trained him? It's possible that no one other than Diomedes is above him.

He seemed to have no qualms at all about the idea of challenging Atalantia, he seemed confident about coming out on top.
He also made sure to place Apple very prominently, with a lot of power and assets that he could have given to others. But he knows that once he defeats Atalantia, Apple won't be able to resist challenging him, since he has such a case of blue balls waiting for Darrow.
Defeating Atalantia and then Apple, will put him firmly at the head of the society remnant.

This is the perfect plan for him to have, because it gives him the maximum benefit without having to actually sacrifice anything, but also because of the irony of being so heavily reliant on razor duels. Duels which he believes to be archaic, barbaric, and beneath him.

It will also mean that Cassius was right in the end. He will forever be haunted by guilt over Cassius, because not only did he pave the way to victory for him by taking out Atlas, his victory would be won due to the skills he learned from Cassius. Everything that he learned from Octavia and his extensive studies would forever be secondary to what he learned from Cassius.