Reread the manga, no he didnt dodge it, the blade scratched Domu and made his head tilt
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Did Deidara use his strongest jutsu against the Sanbi? Did Deidara use his strongest jutsu against Gaara when he was still a jinchuriki? That answers your question.
Yes like I have already proved.
Its a world dominated by humans who uses all kind of summons,animals and monsters to fight alongside with them, once again, it was never specified what kind of enemy, the way he said enemy means any enemy unless they have some kind of armor to prevent the scythe from scratching the skin. Gamabunta's skin is clearly exposed and Hidan's scythe is more than hard enough to pierce through it
it was hard enough to destroy a rocky ground and leave a trail behind just by running with it.
Nope, he just jumped and ran on Gamabunta and was just messing around with him. That, without even shinra tensei, just with basic ninja skills, Hidan can do it too, let alone that he's more agile and swift than Deva Path.