I've been doing extensive testing on Mega Diancie, and I eventually plan to write the analysis for her, but I dunno if I'll be able to. But anyway, my most recent test was the Rock Polish set, and I've found that it works pretty good. You just have to set up at the right time, on something she checks or counters...and Scizor needs to be gone for this to work.
Diancie @ Diancite - Naive Nature (Magic Bounce)
EVs: 152 HP / 16 Attack / 252 Sp. Attack / 88 Speed
move 1: Diamond Storm
move 2: Moonblast
move 3: Earth Power
move 4: Rock Polish
16 Attack EVs allows Diancie to kill Zapdos after Stealth Rock with Diamond Storm. 88 Speed allows her to outspeed Sand Rush Excadrill after one Rock Polish boost, meaning that she then go on to outspeed pretty much everything in the entire tier. 252 EVs are invested into Sp. Attack to increase the power of Moonblast and Earth Power, and the rest is invested into HP to increase Diancie's bad HP stat by a little bit. Diamond Storm is her, really, only good physical STAB, giving her a 50% chance to boost her defense after using it. Moonblast can also lower the foe's Sp. Attack, and Earth Power can lower Sp. Defense. Rock Polish is the main move of the set, doubling Diancie's speed.
Also, you will be able to teach Diancie Earth Power by spending a few BP at the Battle Resort in ORAS. I guess shards are a thing of the past, but hey - now those BP you rack up from Bank are more useful!