The "contempt of plot" item in Wrath of the Titans (well, the most glaring one) involves the plot device spear. Much of the movie is a macguffin hunt, seeking to reassemble a superweapon that was used to defeat Cronos in the great war against the Titans. It is its own source of power, you don't need to be a full god to use it, so Perseus will be able to slay Cronos with it.
But...if it can kill Cronos, why is Cronos still alive as opposed to imprisoned in Tartarus where there's a constant danger of escape?
All they had to do was have Perseus ask this question, and get one of two short answers:
1) "He was still our father, Perseus, we couldn't bring ourselves to kill him."
2) "At the height of his powers, Cronos was merely staggered by the spear. But a few aeons in Tartarus has sapped his strength, the spear is now enough to slay him."