Having defeated your first powerful foe by unleashing a devestating finisher, you continue training for your next battle. This is a really long process, typically spanning many dreams, but Morpheus decides to take it easy on you and simplifies some things.
First, you meet some guys who are really strong and beat you up, so you get stronger to beat them up back. Then they beat up some of your friends, so you beat them up. Then stronger guys come and kill your friends, so you kill them. Your friends get better. This kind of thing keeps happening. One particular little bald friend dies frequently, until he finally grows hair and decides to let you do the fighting. After "decades" of training and fighting, you stand above all other contestants as the the most powerful fighter across an array of intermingled universes.
As the dust settles, you stand before Morpheus, the Dream itself, embodiment of all that could and must be true. You challenge his power. Here, in his world, he transcends omnipotence, but you have dreamed of a special kind of power. Your ability is plot device. You have the power to overcome any opponent simply by necessity.
Even having written the script, Morpheus is unprepared for the twist it has taken as you awakened your inner Saiyan. In the end, he succumbs to you and your infinite potential. As he falls, the realm you inhabit starts to decay, and you awake with the taste of bittersweet triumph on your tongue.