Batman's "no kill" rule is famous in DC Comics, but not even his son, Robin (Damian Wayne) can abide by it, even for Superman. As the child of Talia al Ghul and Bruce Wayne, Damian is a fierce warrior whose lethal tendencies often have to be reined in by other members of the Bat-Family. But aside from his fighting skills, the sharpest weapon in Damian's arsenal is his intellect, which he frequently uses to outsmart even the most powerful of enemies. As it turns out, Damian has a contingency plan that could effectively take down Superman.
In Batman #77, Damian's Robin went up against Gotham Girl, who had been given an equal power-set to Superman (written by Tom King, pencils by Mikel Janin and Tony S. Daniel, inks by Mikel Janin, Tony S. Daniel, and Norm Rapmund, colors by Jordie Bellaire and Tomeu Morey, and letters by Clayton Cowles). While he initially looked outmatched, Damian came prepared, using a wand to cast a spell that trapped Gotham Girl in chains. Pleased with his work, he mused, "Said it before. Super-people are not so hard" before running off.
While Damian didn't directly go up against Superman here, he still made an effective plan against Gotham Girl, knowing that one of Superman's greatest weaknesses is magic. Coupled with the fact that Damian has been trained as an assassin, the ease at which he was able to take down Gotham Girl points towards an idea that he could kill Superman on his own if he felt the need or desire to. Even without the direct use of Kryptonite, Damian's cunning nature could spell trouble for Superman.
The threat that Damian Wayne's Robin poses to Superman is twofold. The combination of his brains and his connections would allow him to take Superman by surprise. As a member of the Bat-Family and Batman's son, Damian has access to all of his father's tech and weapons. Though Damian did not use Kryptonite to stop Gotham Girl, it is likely that he would still be able to use it, given Batman's contingency plans for the Justice League.
While Batman himself would stop short of actually killing Superman, Damian has proven time and time again that the same rules do not apply to him. While his smarts allow him to get the upper hand against his foes, Damian is not compelled by the same commitment to avoid murder at all costs like Bruce Wayne. His relationship to violence is much different from his father's, as his early life was not shaped by a devastating experience of loss. As he proved with Gotham Girl, Damian Wayne's Robin knows how to effectively counter the weaknesses of his foes, a line of thought that not even Superman is exempt from.