if we're talking the real The Fallen then at least equal to Vector Prime's true form and on par with Solus Prime's true form which is in the order of being able to stop calamities that tl;dr would destroy an infinite number of multiverses in shorthand, and Solus shattered the Star Saber which can destroy the TF main multiverse, which contains an endless amount of generating multiverses, and immeasurable speed as the 13 all exist outside of time and space, alongside immortality, resistance to existence erasure, soulfuck on a massive way scaling from Liege Maximo and Nexus Prime, the 13 are kind of a big wall within TF for most characters put up against the setting and outside of the Royal Knights from Digimon I don't know of a single fair matchup for them. But this is all for their real forms, they have avatar bodies that vary wildly in strength, but The Fallen appears so infrequently I'm not aware of any avatars that are a good fight for an Ultraman. Dreamwave and Bayformers...