@TheWatcher: You ask how Bhishma could defeat an incarnation of God, Parashurama. There are many ways we can interpret that.
Bhishma never saw his Guru as a threat to him. We learn God defeated and killed many bad persons. But then what does an incarnation of God (say Rama or Krishna or Parashurama) actually does? Kill the ego within is what God's incarnation does as God is full of love. Through all the fight Bhishma had with Parashu, Bhishma fought because the Guru wanted him to fight and Bhishma had humility for the Guru throughout. All that Bhishma said was he would be steadfast in his oath whatever happened which itself Guru saw as a noble quality. If there is one way, we can win over God that is by humility which is equivalent to devotion or surrender. God sees each of us as capable as Himself as He created each of us that way and so in this case He never sees one bigger or one smaller . This fight is totally different from Ravana fighting Rama or Sisupala fighting Krishna. In those cases, God's incarnation gave the chance of peace which Ravana or Sisupala declined to accept because of their own pride, conceit and ego and so the fight happened in which God destroys the ego within. Interestingly even though it is said Parashu could not win against Bhishma, Bhishma had no inclination to show his superiority. .