While Serpico effortlessly slices and dices the "lesser" creatures he faces, when it comes to larger or more powerful foes his sword proves to be less effective. During the battle at Flora's mansion, Serpico uses the sword to attack an apostle. He is able to blind him (which is his goal), but the sword doesn't slice through the apostle's head.
Now you could argue that since Serpico just tries to blind the apostle he doesn't use the sword's "full strength" hence why the wind slash doesn't slice through the apostle's head. However, if you notice Serpico never uses the sword's power against the apostles before or after that incident, even when he's in severe danger. This implies that either Serpico doesn't believe the sword's slashes alone can defeat apostles or he knows the sword's slashes alone can't defeat apostles. And given the fact he has enough faith in the sword's abilities to try and use it against foes who objectively far outmatch him (Daiba and the Kundalini, for example) I sincerely doubt that the former is the case at Flora's mansion.
I believe this view is reinforced by the group's first encounter with the Makara. Serpico doesn't try to slice it in half or cut off its trunk or tail or legs. He tries to blind it, the same strategy he uses against the apostle. To me this implies that in both cases Serpico knows he can't outright slice through them like they're trolls or Daka so he tries to hinder them instead. (Also it's worth mentioning that not only can Serpico not slice through the Makara but it's faster than him as well, counterattacking before Serpico can blind it and nearly killing him in the process.)
With the exception of the Sea God and arguably the Daiba/Kundalini duo, Zodd is far faster, stronger, and tougher than anything Serpico has faced. Zodd could take multiple shots from Ganishka's lightning, which has the power to kill the weakest apostles almost instantly. If Zodd can take that kind of punishment from Ganishka's lightning then Serpico's sword's powers probably wouldn't cause him too many problems, much less decapitate him with one slice.