Age 777...
There was only a single year before Lord Beerus was to be awoken for his fight against Kakarot, and as always, Kakarot and Whis were training. Taking a break for lunch, Whis asked Kakarot, "It's almost time for you to fight Lord Beerus Kakarot. Are you nervous?"
Kakarot shook his head and after swallowing a bite of his lunch, he told him, "No, but I am excited. You've trained me most of my life. You've told me I'm fighting for my life in this fight, but even that doesn't scare me. You didn't spend thirty nine years training me only for me to lose. I will win this fight, whatever it takes."
Whis smiled, "That's exactly the attitude you'll need if you wish to defeat Lord Beerus."
Kakarot realized, "If this is a fight to the death, what will happen if- no, when I defeat Lord Beerus?"
Whis chuckled lightly, "Do you want to spoil the surprise?"
Kakarot chuckled back, "No, I'd like to wait until it's all said and done."
Whis believed that Kakarot knew exactly what would happen if he were to kill Beerus, that he would be his replacement.
After finishing their lunch, they continued to train for the rest of the day.
One Year Later, Age 778...
It was time, after 39 years, Beerus was groggily waking up. When he came to his senses, he got up and exited his castle, seeing Kakarot ready to fight, already in his Super Saiyan God form. He smirked, "Well, aren't you ready for a fight Saiyan?"
Kakarot nodded, "This is what I've been training for. For the last thirty nine years, I've been training, and now I'm eager to see if I'm powerful enough to defeat you!" before powering up and dashing at Beerus to punch him in the gut.
Beerus was caught off guard by Kakarot's speed, and was actually recoiling from the blow to his gut. He wound up a punch, only for Kakarot to continue to attack, not giving him an opening to attack. "You're good Saiyan, I wouldn't expect any less from a student Whis trained most of his life."
Kakarot raised an eyebrow and asked, "Are you just going to talk, or try to fight back?"
Beerus growled, "Not afraid to anger a God of Destruction, are you?" before sending a multitude of energy blasts at Kakarot.
As the battle continued, Kakarot asked Beerus, "Just wondering, you're not holding back are you? This isn't any fun. I'll admit, I'm only using about 25% of my full power, Lord Whis specifically asked me to start at 10% and work my way up. He said you would go all out, but if this is all out for you, I'm disappointed."
Beerus cringed when he heard Kakarot was holding back, especially when Kakarot told him he was using only 25% of his full power, Beerus himself was using 40%, and was still having trouble doing any real damage to Kakarot. "You're holding back!? All right then, let's see how you do at this speed!" before trying to attack again
The fight continued for hours, almost an entire day, and Beerus had finally gone as far as to use his full power. Whis sighed as he watched the battle, "It seems Lord Beerus is going all out now, but Kakarot isn't." before yelling up to them, "If either of you intend to end this fight, please do it now!"
Beerus growled, "Whis is right, let's finish this!" before charging up a massive energy blast and throwing it at Kakarot. He thought to himself, "He's still holding back, so he might not be able to tank this one. I may have created my own undoing." before feeling a sharp pain in his chest. Kakarot had sent a thin but precise finger beam, not only nullifying his energy blast, but sending it through his body, he chuckled as he fell into unconsciousness, "Well Kakarot, it seems I underestimated you. You've become too strong for me." before falling to the ground on his planet and passing out, shortly dying afterwards.
Whis looked up to Kakarot and said, "It seems you've won. You remembered this was a fight to the death, and I won't lie, I wasn't sure you'd win. But you did. And now, you're no longer just a simple apprentice." before bowing respectfully and saying, "You are now Lord Kakarot, the new God of Destruction of Universe 7." before waving his staff and giving Kakarot the same cloths that the now late Beerus had worn.
And so, the fight that Kakarot trained his whole life for has come to an end, with Kakarot not only defeating, but killing Lord Beerus. And with Beerus's death, Kakarot takes his place at Universe 7's God of Destruction. But this is not the end of Kakarot's story. In fact, it is only just the beginning. Lord Kakarot knows there are things in Universe 7 that need to be dealt with, and he is the only one powerful enough to do so.