Most video games lead up to a final boss fight. These battles are meant to be the culmination of your adventure; the final climactic moment where you take all of the skills you've learnt over the course of the game and put them to use in one ultimate clash with a big bad foe.
Many game antagonists are built up over the course of the story, often demonstrating immeasurable power or performing despicable acts to make you fear and hate them more and more. As time goes by, you can't wait to get to the end and finally serve them a dose of well-deserved justice. You go into those battles with a mixture of excitement at the thought of bringing them down, and apprehension about how tough they might turn out to be.
But not every boss fight justifies its build-up. Some bosses are made out to be the toughest, nastiest beings in the known world, but actually end up being ridiculously easy to beat.
What you thought was going to be the fight of your life actually turns out to be a major anti-climax; some of the weakest end bosses in video game history.