Krishna killing Kansa. Source: Wikimedia
Shri Krishna of Dwarka and Other Stories, by C. A. Kincaid.
Krishna was almost ready to become a man. He had bested many beasts so far, and to him, it looked like no one could challenge his might. Krishna began to hear tales of a mighty beast in the forest that was supposedly stronger than any he had faced. He set out immediately to challenge the beast and test his strength. When he arrived in the forest he explored around for a bit until he came upon the beasts path of destruction. Massive trees were snapped in two like twigs along the warpath of this voracious beast and there wasn't a living being in sight. Krishna followed the path and eventually caught up with the raging beast. When he arrived at the end of the trail he found a gargantuan Bull that was feasting on the creatures of the forest. Once the bull noticed Krishna it charged straight for him. Krishna, without missing a beat, grabs the bull by the horns and launches it 100 meters away, snapping a horn in the process. Krishna then takes the horn and beats the beast with it until it is a pile of pulp, unrecognizable from its previous form. Krishna became sad because he thought this beast would prove to be a worthy challenge, but it ended up being just like all the rest. He accepted that there might not be any being that could challenge him and went home.
King Kansa, in the meantime, was making preparations to send more assassins and monsters to send after Krishna. He stopped his preparations when he heard how Krishna had easily beaten the demon bull in the forest. Kansa didn't even send that monster after Krishna and it was much stronger than anything he had in his arsenal. Kansa and his consort decided that the best way to deal with Krishna was to lure him to a trap in the form of a grand wrestling tournament where he would summon all of his strongest men to take down Krishna all at once. They thought that they had better deal with Krishna soon before he becomes a man and no one can challenge him.
Krishna hears of the tournament and is aware of the plot, but still decides that he must go. He arrives in the city and many fall at his feet worshipping his glory. Krishna enters the arena and is immediately attacked by all of King Kansa's strongest men. He beats them all with ease and turns to face Kansa as all of the spectators start to cheer and celebrate his great victory. Krishna leaps up to Kansa's high throne and drags him to the arena. Kansa, knowing his fate already, gives a pitiful attempt to defend himself and perishes. Krishna had fullfilled Kansa's prophecy, even though Kansa had tried everything in his power to prevent it.
Author's Note: I decided to simplify this story a bit. I tied Krishna's fight with the bull into his fight with Kansa because it flows a little better and makes more sense that they would be related. I also removed Balarama to make it more about Krishna vs. Kansa.