Mega diancie moving down to A is a little ridiculous, imo. I've used quite a lot, and it is an absolute monster. In reality, it has very few flaws, it's just that those flaws are pretty major.
Cons:
-really bad defensive typing
-50/110/110 defenses aren't as great as they sound
-horrendous speed pre-mega
-has to go mixed to use both stabs
Keep in mind that that's really not very much at all, the thing is, they are pretty severe. However, it's numerous positive traits greatly mitigate this.
Pros:
-great speed tier
-great offensive stats on both sides
-magic bounce
-wonderful offensive typing.
-decent bulk for an offensive mon
-multiple unique sets that all perform very well against an archetype.
With these in mind, I'd argue that diancie is almost justifiable for S rank (ALMOST, not quite). It's most common set, protect+3 attacks, is great for the ability to scout for stuff like bullet punch and it lets it mega evolve easiest, and is, for this reason, perhaps its most reliable. However, often people will be caught off guard by its other two most common sets, namely RP and CM. Thanks to Magic Bounce, it's CM set destroys Stall, as many don't have the ability to muscle past it on the special side after a boost or two, and those that can break it on the physical side are usually dead before they can. It's pseudo-immunity to status makes it easy to set up on stall teams and really hard to stop once it is going. The last is rock polish and, while not always the most reliable, is probably the most deadly against offense. Even scizor and ferrothorn aren't even entirely safe against it, as HP fire is actually a very viable option (though it isn't usually preferable).
Tl; dr, mega diancie is a very dangerous mon w/ S rank pros, but a few major flaws that limit it from performing at the same level. However, it fits A+ perfectly.
Cons:
-really bad defensive typing
-50/110/110 defenses aren't as great as they sound
-horrendous speed pre-mega
-has to go mixed to use both stabs
Keep in mind that that's really not very much at all, the thing is, they are pretty severe. However, it's numerous positive traits greatly mitigate this.
Pros:
-great speed tier
-great offensive stats on both sides
-magic bounce
-wonderful offensive typing.
-decent bulk for an offensive mon
-multiple unique sets that all perform very well against an archetype.
With these in mind, I'd argue that diancie is almost justifiable for S rank (ALMOST, not quite). It's most common set, protect+3 attacks, is great for the ability to scout for stuff like bullet punch and it lets it mega evolve easiest, and is, for this reason, perhaps its most reliable. However, often people will be caught off guard by its other two most common sets, namely RP and CM. Thanks to Magic Bounce, it's CM set destroys Stall, as many don't have the ability to muscle past it on the special side after a boost or two, and those that can break it on the physical side are usually dead before they can. It's pseudo-immunity to status makes it easy to set up on stall teams and really hard to stop once it is going. The last is rock polish and, while not always the most reliable, is probably the most deadly against offense. Even scizor and ferrothorn aren't even entirely safe against it, as HP fire is actually a very viable option (though it isn't usually preferable).
Tl; dr, mega diancie is a very dangerous mon w/ S rank pros, but a few major flaws that limit it from performing at the same level. However, it fits A+ perfectly.