I wasn't sure about Mega Diancie being broken until I tested it myself. In theory Mega Diancie isn't that good because it's slow and frail the turn it Mega evolves, but in practice is incredibly easy to find opportunities to threaten the opposite team and / or setup. Mega Diancie can run Protect + 3 attacks to guarantee its Mega Evolution, but it can also use Pokemon such as Chesnaught, Crobat and Umbreon to do it without needing that move, allowing it to add Rock Polish to its moveset. After getting its mega evolution turn out of the way, there's not much that can stop Mega Diancie.



160 base Attack and Special Attack is huge in the UnderUsed tier, and that combined with 110 base Speed is a bit ridiculous. Moonblast, Diamond Storm and Earth Power have great coverage in the tier, nothing resists that combination, and allow Mega Diancie to hit extremely hard from both sides, which is huge because Diamond Storm with minimal EV investment 2HKOes dedicated Special walls. It is fast enough to outspeed a huge portion of the metagame with a +Speed nature, but it can afford running a +Special Attack nature on the Rock Polish set and still be able to outspeed every single scarfer at +2.



There's only a handful Pokemon that aren't 2HKOed by Diancie and none of them is particularly common in the UnderUser tier, talking about Pokemon such as Weezing, Bronzong and Physically Defensive Amoonguss. Stall struggles against Mega Diancie's ability to wallbreak the tier's most common walls, and Magic Bounce alone means using hazards to wear it down can easily bacfire. More offensive playstyles, including balance, lack the ability to repeatedly switch into Diancie's move, if it's running a +Speed nature revenge killing it without a Scarf Pokemon is quite hard. And lastly, offense and HO basically lose if its gets a Rock Polish off.



Considering all these things I believe it should be banned, there's really not many ways of playing around Mega Diancie and it's a huge threat to every single playtyle without even needing any specialized support.