Sephiroth does not seem like the guy who roll up his sleeves and impale someone as a symbol of strength. Terryc250 is right, that a limit to Sephiroth's strength can not be determined. And although we have never gotten any solid fact of how physically strong he actually is, we can not conclude that he might not have the physical capability to lift the serpent.
However, we can also not conclude that he has it. Because as Terryc250 said, it was never specified how the serpent became impaled. Assuming it was impaled trough physical strength is just as wrong as assuming it was not. We do not know how the battle between Sephiroth and the serpent in question took place.
It could have been an accident. It could have been pure skills from Sephiroth. It could have been injured while at high altitude and fallen over the tree. It could have been blasted there by Sephiroth. It could have been dragged on top of it by Sephiroth after defeat. It could have been thrown there during combat by Sephiroth.
Heck, considering the tiny amount of facts there is revolving that thread, it might not even have been Sephiroth, or the serpent was maybe tired of living?