Again, the Argentiums in general.

They're one of the few groups in Unova that's well-versed in Aura and Martial Arts (both of them are intertwined since Aura is also referred to as "Ki" in the Murim). Typically, Elio and the rest spent their entire lives honing their respective Auras and martial prowess - both of which have been proven to have beneficial effects on the body when honed properly, so they have a damn good handle on it. However, it doesn't mean they're invincible, as Aura mastery has grades and people of higher levels naturally exist.

If you wanna think of Aura in another way, think cultivation from Wuxia.

This extends to their Pokemon as well, since Pokemon are more potent Aura users than humans - which would partially explain why an Argentium's party members are unusually strong, even by veteran standards.