The mage who walks in the heavens
(Newbie's work, description is weak, if you don't like it, I'm sorry)
Traversing other worlds, cultivating magic, and conquering other planes, he discovered that every plane was a familiar world. Marvel,(CD, anime, Hong Kong, 2D, none of these currently available), and so on…
"Hmm? Doctor Strange wants to borrow my power?" "Agreed."
"What, Harry Potter got beaten up by Voldemort, and you want me to come over and support him?"
"Gandaph, don't be idle, go help!"
"What is it? The Holy Master ran out again?"
"Ghidora, go and exchange a few moves with the Holy Master. If you lose, don't come back."
Merlin looked at the messages sent by the worlds under him and could not help but sigh.
"It's really tiring to be the big brother of the myriad worlds. I want to retire!"
This was the story of a mage walking in the heavens.
Goku's body had a base power level of 75 trillion when he was fighting frieza pre-golden and this timeline is a couple of years after the universe 6 and 7 tournament. I'd estimate Goku's body in base should be at around twice as strong, so 150 trillion. Goten had a power level of 40 million at the start of Super, being generous and say he's had light training with Trunks and some from Chi-Chi as she did train him somewhat during dbz before the start of the Buu saga, Goten may be at 80 million in base, slightly stronger than Goku's base power when he first fought Beerus at the beginning of super. Now, SS1 and SS2 are a multiplier of x50 and x100 respectively, going up to x62.5 and x125 in their grade 2 variants, therefore, Goten's power level at SS2 would be, 80,000,000 x 100 = 8,000,000,000/8 billion. Even if he pushed to grade 2, his kamehameha would have done squat, even if we gave him a rage boost similar to the one Gohan had against Frieza on Namek a x5 boost, 80,000,000 x 125 x5 = 50,000,000,000/ 50 billion. Goku Black stole the body of a Goku that was a few years ahead of the universe 6-7 tournament, during which his base power level was 85,000,000,000,000/ 85 trillion, say after the extra years of training after the destroyers' tournement, Goku is at 150 trillion in base, Goku Black would be tanking Goten's blasts with literal ease as he's 1700 times Goten's full power. In the Dragon Ball world, it's stated that you need a 20% gap in power to easily beat an opponent not taking into account, technique in martial arts or hax abilities like time skip, kaioken and the like, Black being that much more powerful make's Goten a joke in comparison even if he wasn't taking it seriously. Our Goku from the present timeline managed to get 10 times stronger inbetween the Golden frieza and Zamasu saga' so the body Black stole could have definitely gotten to 150 trillion in base in a few years of training.