Well, I don't think being 'unkillable' counts in this sence, it isn't a Jason sort of deal, where he takes fatal damage and keeps on going, I'd assume he dissipates or the like, and I doubt he comes back frequently enough to wear Zed down.
There is also the fact Zed is a ninja, so you'd have to assume he has heightened senses outside of powers given by his magic as well as being alert for any other enemies. So Pyke, making any noise, smelling of sea water, anything would give him away.
And as stated, Zed is likely more skilled in combat, thus in the long game Pyke might win, when Zed is old and decrepit. At least that would be my take on it. Best scenario for Pyke is the fight draws attention to Zed where as his enemies can get an upper hand on Zed.
Also, yes it is in game, but Zed is one of the only, if only, champion that never truly dies, he shadows out, maybe dying, or maybe escaping. Feel it's worth noting.