Flash is definitely very strong, but he is nowhere near a match for Asta. The kid has a lot of incredible abilities and has been getting steadily stronger with each arc. Flash cannot hope to keep up with this lead’s growth and I wouldn’t be surprised if Asta could beat him with a single blow at this point. At the end of the day Asta is barely human anymore while Flash is certainly mortal. Asta wins.
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Flash Gordon has been superseded by most anime characters, so he has no chance of winning this. Even Neena Trinity could beat him and still have time to be sipping mimosas with Revy and Maria Traydor (Star Ocean).
Yeah, Flash definitely can’t compete with the big guns