After the Swords of Justice intervene to prevent any further injury to Keldeo, he snaps at them for interfering and uses Ice Burn on them, freezing them solid. To repeat, he took out the Musketeer Trio, 3 extremely powerful Legendaries with immense combat training, with a single attack.
And despite Keldeo's best efforts the guy still won the rematch.
Also, Kyurem is referred to as the ''world's strongest dragon-type'', which might be stretching it (considering the likes of Dialga and Palkia are also dragon-types) but is nonetheless a small glimpse of how goddamned powerful this Pokemon is.
Kyurem's method of accepting a challenge is to create a massive ice arena with a front gate made of icicles, which the challenger must pass through to start the battle. As if anyone needed further proof that Kyurem is a badass.
A minor one: when Keldeo returns to Kyurem and admits that he lied about his Sword of Justice title, Kyurem then reveals that he knew the whole time, and just wanted to fight him anyaway.
Kyurem freezing the remains of his den as the building starts to collapse all around our heroes. Kyurem proceeds to create an ice castle in the process.
Of course, special mention goes to Keldeo's rematch against the Boundary Pokemon. After overcoming his fear and transforming into Resolute Form, Keldeo proceeds to show Kyurem just how powerful he's become, with barrages of Focus Blasts, even faster speed, using Double Team and the environment to great effect, and of course breaking out of solid ice using his newly-learned Secret Sword.
Of course, Kyurem is no pushover, using his White, Black, and normal forms to deliver various kinds of heavy bombardment. While his devastating attacks have Keldeo on the run, he tanks everything the Colt Pokemon throws at him, all the while laying waste to everything around him. While he might not have Keldeo's finesse, his durability and the raw destructive power of his attacks is still something to awe at.
After Keldeo loses to Kyurem due to his Heroic Sacrifice to protect Ash and co. from Kyurem's misdirected Freeze Shock, he is forced to yield. Kyurem's response?