Jiren of Universe 11's Pride Troopers is a one-man wrecking machine in Dragon Ball Super. The primary antagonist of the Tournament of Power, though not a traditional villain, Jiren lived in Goku's head rent-free after their first bout left him twitching in a pile of rubble. The Pride Trooper's strength is so absolute that even his own universe's God of Destruction freely admits that Jiren is far beyond him in power. Considering that Gods of Destruction set the new bar for power levels in Super, the question on every fan's mind during the Tournament was the same: how did Jiren get so strong?
Jiren spent the Tournament of Power either meditating for lack of worthy opponents or taking the multiverse's strongest warriors and beating them like cheap drums. With no transformations or gimmicky techniques, just hands rated E for everyone, he needed only a single finger to defend against Goku's Super Saiyan God and bodied Goku's Super Saiyan Blue Kaioken x20 before grinding the mighty Saiyan into the dirt. Jiren's power is so outrageous that -- even with Super's absurd power scaling -- it took Goku achieving Ultra Instinct, the power of Angels, for Jiren to actually exert himself.
Belmod, Universe 11's God of Destruction, as well as Marcarita, his attending Angel, have been training Jiren for an unknown length of time. Considering Goku's and Vegeta's power levels after their relatively short time training under Whis, it's likely that Jiren has been training with his own universe's deities for far longer than our heroes. While Jiren does take orders from Belmod, he is not nearly as formal with him as Gods of Destruction typically demand from mortals, indicating that Jiren and Belmod both know who would win should they come to blows.
Training with a God of Destruction is a great way to achieve immense power, but that alone shouldn't be enough to actually surpass that Destroyer in strength. For Jiren, tragedy is the true motivator of his endless quest for power. Unlike Universe 7, where our heroes reside, there are no Dragon Balls in Jiren's universe. In Universe 11, death is permanent and Jiren has been molded by the pain of losing everyone he has ever loved or respected and being betrayed by those who remained.
When Jiren was a child, an "evil-doer" attacked his village and murdered his parents. Jiren was found by a man who trained him for years and when the villain eventually returned, Jiren's master was killed and his fellow warriors refused to fight with him out of fear of the faceless monster. Having lost everyone he loved and abandoned by his comrades, Jiren committed himself to a solitary existence in pursuit of absolute strength, all so that he would never lose or need the help of others ever again.
These days in Dragon Ball Super, Vegeta has become a family man and even Goku remembers to visit his own family from time to time, while Jiren has nothing and no one. With no reason to ever halt his training and lingering grief that wish-spammers can't relate to, it's plain to see how Jiren has become so powerful. Jiren's strength comes from his suffering, something no one in Universe 7, other than Future Trunks, could ever relate to. Jiren, however, took his grieving to the extreme by training relentlessly instead of trying to make peace with his pain.
Jiren has both nothing and everything to lose when he fights, giving him both honor and rage. He has no loved ones to protect, but he doesn't enjoy causing others pain because he knows what it is to lose someone. However, his fragile psyche is on the line in every battle, and he will do whatever it takes to avoid feeling the way he did when his family and master were murdered. This contradiction inside him makes him nearly unstoppable, but he becomes anger incarnate when he's cornered. While rage has never gotten Vegeta very far on the battlefield, it's done wonders for Jiren.