Mega Diancie? God, if there's one mega I'm excited to use right now it's that thing.
It got nothing but straight-up buffs. Dugtrio's existence kinda hurts it, but I doubt that would remain a thing for too long.
If that thing isn't A+ or even S idk what to say. From being able to speed tie with fellow 110s while running HP Fire to getting +2 defense half the time from Diamond Storm to getting Power Gem for more special sets to no longer being forced to run Protect, letting Sharpen, CM, and RP variants run amok with far better coverage, literally everything but a metagame trend or two is going for this thing.
I said it when SuMo were released, so long ago, and I'll say it again: Mega Diancie is going to be huge in this meta. Mark my words.
Actually, I think it's much worse this generation. HP Fire simply isn't strong enough to break through the extremely bulky steels this meta has. Sp. Def Celesteela and AV Magearna wall it to hell and back. Earth Power also doesn't hit many of them hard enough if at all, like Ferrothorn, Skarmory, Mega Scizor and Celesteela. This is in addition to many more offensive threats that can break it, such as Greninja, Tapu Bulu, and most notably, Rain, as Mega Swampert doesn't go too light on it. Also, Power Gem is outclassed entirely by Diamond Storm:
Diamond Storm will almost certainly be better than Power Gem. For starters, it simply is the more powerful option:
252 SpA Diancie-Mega Power Gem vs. 0 HP / 0 SpD Mew: 153-181 (44.8 - 53%) -- 28.9% chance to 2HKO
4 Atk Diancie-Mega Diamond Storm vs. 0 HP / 0 Def Mew: 163-193 (47.8 - 56.5%) -- 86.7% chance to 2HKO
Since Diamond Storm is so powerful in of itself, the lack of physical investment doesn't really matter. It also comes with the general merits of running a mixed attacker, like hitting special walls harder:
252 SpA Diancie-Mega Power Gem vs. 252 HP / 252+ SpD Celesteela: 109-130 (27.3 - 32.6%) -- 77.1% chance to 4HKO after Leftovers recovery
4 Atk Diancie-Mega Diamond Storm vs. 252 HP / 4 Def Celesteela: 159-187 (39.9 - 46.9%) -- guaranteed 3HKO after Leftovers recovery
Add that to the fact that Diamond Storm got a buff this gen, giving it a 50% chance to double its defense, and there is almost no merit to running a fully special Diancie.
As for Diancie itself, gen 7 has turned out to be quite a mixed bag for it, as the rise in steels (primarily Magearna) pressure it considerably, the new offensive fairies this gen add increased competition for its role, and the low usage of Talonflame and Tornadus-T means it loses out on a lot of the merit for its typing. However, the new mega mechanics free up its final move slot, making it super hard to wall, and it still has some defensive utility, like checking Mega Pinsir. Add that to it just simply being so strong and fast and I'd imagine it to have quite a successful run in gen 7. Can't see it going lower than A or A-.
This post also notably points out the fall of Bird spam. Dark spam has also fallen off, and this really hurts its niche. I can see that both Earth Power and HP Fire might be necessary this generation because Rock/Fairy is just very easily walled. I'd also say it requires Dugtrio support alongside this. Even then, it's offensively dealt with quite easily due to the large number of threats it cannot switch into. Previously, it had decent bulk and a passable defensive typing that notably let it switch into Flying and Dark type moves. Now, it still has ok bulk but with Ground and Water moves even more prominent than last generation, it now essentially relies on a free switch-in.
You thought about it in a complete vacuum. You didn't account for what around it has changed, (other than the wise observation of Dugtrio, which you only thought one side about), which is literally the most important part lol. Think about the meta around it. It's a lot worse for it. Don't just theorymon lol.
As a result of its typing being much worse, it now requiring 2 moves to hit steels and there being more to offensively check it, I believe that Mega Diancie is nowhere near as good as last generation, and will likely find itself in A-, much less S lol.
That being said,
I hope mega Diancie is hot trash in this meta because Koko, Magearna, and Bulu while having less defensive utility but fairy spam is pretty dumb and it still has immense presence vs fucking hell.
My only wish is that it's like Mega Lopunny, both do great vs offense but struggle vs balance.
I don't think that it'll be
trash, though. Magic Bounce is still godly for it, and it's still incredibly powerful. Rock/Fairy is walled by many common things but it hits a ton of things super effectively, most notably Fire and Dragon. Magic Bounce in particular is a huge boon because it lets it serve as some form of hazard control - not a reliable form, but
some form - for offense, which will appreciate not having to fit in a Defogger. That being said, I still think that it's not very good in the meta, but it's not complete garbage.
Also, Mega Lopunny can do well against balance if you use PuP + Encore. If your team struggles vs offense, it can go for Fake Out + Quick Attack. I think that Mega Lopunny is somewhat slept on in this current metagame because while it is set-dependent, it can screw over a lot of teams.