>>51502422The biggest threat to Diancie is Thanatos due to its ridiculous stats, typing, and having Earthquake. It helps to have a fren for this one. Breloom can switch in, tank a hit, outspeed and put it to sleep, and 2HKO it with Superpower - Diancie can revenge kill after if it wakes up early, and then continue its rampage.
>252+ SpA Diancie-Mega Earth Power vs. 0 HP / 0 SpD Tyranitar-Mega: 216-256 (63.3 - 75%) -- guaranteed 2HKOFor options, you can also throw SR on Diancie instead of HP Fighting if you want the extra IVs or don't want to grind. Earth Power is also expendable. It's up to you. Since I didn't mention it earlier, Earth Power is a guaranteed 2HKO on Bisharp, and hits a bit harder than Moonblast on Thanatos when he's Mega Evolved.
>252+ SpA Diancie-Mega Earth Power vs. 252 HP / 252 SpD Bisharp: 248-294 (74.2 - 88%) -- guaranteed 2HKOFor support, you'll want something that can set up Psychic Terrain so Diancie can sweep without getting chipped away at by Dark-type priority STAB. Indeedee is great for this: it gets Psychic Surge to automatically set it up upon switch-in, and makes everything unable to use their Dark-type STAB for five turns. You're essentially turning Kronos's Mary Sue ability against his own team, which is fucking hilarious. Definitely a better option than Drizzle Pelipper.
Your wiggle room is limited since you have the rest of the E4 and the Champion to deal with, but Garchomp (who also has 600 BST) can do some cleanup work with Outrage or EQ if Diancie-Mega doesn't make it all the way through Grim's team, and also sweeps Dorado (as mentioned by
>>51500083). WishPass Espeon can be a good cleric, fulfills the role of a backup Magic Bouncer, and takes good advantage of the Eclipse/Psychic Terrain combo.
tl;dr Diancie-Mega is a fantastic sweeper and should be taken advantage of for Grim if you can get your hands on it.
Also, get your hands on some Sacred Ash beforehand in case Grim murders your Pokemon. (2/2)
