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Gee it's almost like having no hard counters doesn't mean shit if you have a shit ton of strong checks, including some of the most common pokemon in the game like Scizor. I mean look at Hydreigon. Last gen it had no counters, and it was pretty shitty.
It's not like M-salamence which has no counters and almost no checks as well that takes a team effort to stop.
This is the best M-Diancie set
228atk/252spAtk/28spe Rash nature
earth power/diamond storm/moon blast/rock polish