To be honest, I don't think the mixed attacker is the best set M-Diancie can use. My thoughts would be that it should do something akin to double-dance P-Z with a set like this.
Diancie @ Diancite
Modest
Clear Body > Magic bounce
156 HP, 252 SpA, 4 SpD, 96 SPE
Calm Mind
Rock Polish
Moonblast
Earthpower
This set after mega evolving outpaces Adamant Lucario (which I think would be a reasonable benchmark to creep without speed boosts), while outspeeding every scarfer after a rock polish. All other EV's to bulk. Depending on how late in the game you are at or what kind of team you're up against, you can use the relevant boosting move of choice (or even both if circumstances are fortunate) and proceed to clean up. I don't really think that diamond storm is that necessary IMO, as earth power + moonblast hits everything for at least neutral damage except for
-Bronzong: Who you can't beat regardless
-Rotom-H: Who cannot do a thing to you except pray for hax from using discharge
-Crobat: Who only wins if carrying a poison move or (lel) steel wing
You can't beat blissey with a mix set anyway, so lets not bother with that. Partner this set up with pursuit trapper krookodile, and you should be able to set up a few win conditions easy.
A noteworthy advantage of modest would be these calcs.
252+ SpA Diancie Earth Power vs. 252 HP / 252+ SpD Filter Mega Aggron: 157-186 (45.6 - 54%) -- 94.1% chance to 2HKO after Stealth Rock
252+ SpA Diancie Earth Power vs. 252 HP / 252+ SpD Empoleon: 180-214 (48.3 - 57.5%) -- 95.3% chance to 2HKO after Stealth Rock and Leftovers recovery
252+ SpA Diancie Earth Power vs. 252 HP / 252+ SpD Jirachi: 182-216 (45 - 53.4%) -- 35.5% chance to 2HKO after Stealth Rock and Leftovers recovery
With Stealth Rock down, you can defeat most of your would be checks, while boosting up against other would be switch-ins.
What I've theorymonned people might like to give a try. Outside of blissey, Banded/scarfed Crobat, bronzong and SpDef hippowdon, there aren't too many good defensive answers. Offense has it better, though if both speed and SpA are boosted, crobat and bullet punch lucario are your only hopes (well maybe medicham with bullet punch, but thats about it. Nothing else in UU should be running bullet punch outside of those two mons).
EDIT: Whoops, stupid damage calc forced me to manually change a few stats since it doesn't have M-Diancie registered, and I forgot to edit the defense stats. If you saw the swampert/hippo calcs, ignore those.