I think Giratina-O might end up being a good fit. With weaknesses to Ice, Fairy, Dark and Dragon, it's not like this thing is going to be a Mega-Sableye tier staller, but it has the bulk to give Megazard, Landorus and Serperior a little less presence as offensive powerhouses. The problem with these guys (and Manectric as well) is that they can reliably chip away at your team without much fear of having to take damage or switch out (fog/spin support for Zard isn't that hard). Unlike other checks to these strong special sweepers, Giratina can actually provide some offense that forces them out. Chansey, the next best option, is a notorious "sitting duck" on anything but stall teams, while Heatran, Skarmory or Tyranitar are either picked off by one of these poke's coverage moves or unable to deal with other strong attacking types like Water/Fighting. Giratina handily prevents Fire/Ground/Fighting/Water from ripping your balanced team apart without its fair share of downsides.
Being weak to Knock-Off AND having an item that's more or less necessary is a minor death sentence, given the move's popularity, and Giratina has little recourse for stopping the tier's Pursuit trappers (Tyranitar, Bisharp, Scizor, etc.) From swapping in on one of its STABs.
Wall of calcs of Dark Types vs. Physically defensive Gira:
252+ Atk Bisharp Knock Off vs. 0 HP / 248 Def Giratina-O: 182-216 (41.2 - 48.9%) -- guaranteed 3HKO
252+ Atk Life Orb Bisharp Knock Off vs. 0 HP / 248 Def Giratina-O: 237-281 (53.7 - 63.7%) -- guaranteed 2HKO
252+ Atk Bisharp Pursuit vs. 0 HP / 248 Def Giratina-O: 222-264 (50.3 - 59.8%) -- guaranteed 2HKO
252 Atk Tyranitar Crunch vs. 0 HP / 248 Def Giratina-O: 212-252 (48 - 57.1%) -- guaranteed 2HKO after sandstorm damage (note: this is = pursuit on switch)
252+ Atk Choice Band Scizor Knock Off vs. 0 HP / 248 Def Giratina-O: 186-220 (42.1 - 49.8%) -- guaranteed 3HKO
0- Atk Mandibuzz Foul Play vs. 0 HP / 248 Def Giratina-O: 254-302 (57.5 - 68.4%) -- guaranteed 2HKO
4 SpA Mega Sableye Shadow Ball vs. 0 HP / 8 SpD Giratina-O: 152-180 (34.4 - 40.8%) -- guaranteed 3HKO
And that's not to mention that one of Giratina's STABs is bait for the strongest attacking type we currently have: Fairy. Imo this thing helps to make up for a lack of offensive Fire/Ground/Water resists in the tier and should fit in nicely without being overly centralizing.
Alright, I'm not gonna respond to all of this, cause I've dealt with a lot of this in prior posts. However, there's a few mistakes I keep seeing popping up that I want to deal with.
First off, Giratina is not weak to Knock Off. Or rather, not as weak as other mons. A lot of people seem to think that Knock Off always gets its damage bonus on every hit because Griseous Orb can never get knocked off (not necessarily saying that's true in your case, robofiend). That's not how Knock Off works. Since Griseous Orb can never be knocked off, Knock Off
never gets bonus damage, just like with Mega Stones.
Second, as previously noted, Griseous Orb is far from useless as an item. It functions as a Spooky Plate AND a Draco Plate, which can never be Knocked Off (and prevents Knock Off's bonus damage). As someone noted (too lazy to check who right now), if you factor this permanent damage bonus into stat values, Giratina actually has base 154 in both offensive stats while attacking with its STAB attacks. And there's no really significant drawbacks to this power either, other than the inability to run Lefties, except even defensive sets benefit from this power, because it lets Giratina hit HARD even with no investment, making it an extremely potent wall that will still be able to hit hard.
Third, Bisharp and Tyranitar (and to a lesser extent other Dark types) are not good counters for a Pokemon that commonly runs Aura Sphere, regardless of how much damage they can do. Not only that, but Giratina doesn't have to run a huge amount of Speed EVs to outspeed Adamant Bisharp, either.
As for those other "counters" you mentioned, yeah, they can do a bit more damage than other Pokemon against that variant of Giratina. Let's see how they do against an offensive version:
252+ SpA Griseous Orb Giratina-O Shadow Ball vs. 248 HP / 8 SpD Scizor: 195-229 (56.8 - 66.7%) -- guaranteed 2HKO after Stealth Rock
If a specially offensive Giratina hits Scizor on the switch, it's not gonna live long enough to take it down, as Giratina will outspeed it, and Giratina probably won't be weak enough for Bullet Punch to finish it off.
Let's see how M-Sable fares.
252+ SpA Griseous Orb Giratina-O Draco Meteor vs. 252 HP / 0 SpD Mega Sableye: 235-277 (77.3 - 91.1%) -- 25% chance to OHKO after Stealth Rock
Even if Sable survives, and factoring in the SAtk drop, Sable's gonna die before Gira does.
Even Gira's Shadow Ball does more to Sable than Sable does to Gira.
252+ SpA Griseous Orb Giratina-O Shadow Ball vs. 252 HP / 0 SpD Mega Sableye: 144-171 (47.3 - 56.2%) -- guaranteed 2HKO after Stealth Rock
And woe be to them if it's a SubCM version and Gira sets up a sub on the switch, or if it's a CroGira version.
Your calcs actually prove the point I've been trying to make. There's plenty of stuff that can counter a given Giratina set, but very VERY few things can counter all of them. The one concession I will make after doing those calcs is that Mandibuzz is actually a pretty good counter for most Gira sets, making it one of the few I've come up with so far that can actually fo that. But Mandibuzz is itself fairly easy to counter, and most teams will probably have a counter for it without even trying to, simply because it hates SR, Fairies, and Electric types. So, I'll give that one point to you, but even a half-decent team still probably won't be troubled by it.
Fourth, I'd also like to point out that given how well Gira goes with MegaGross, I think you're overestimating how much impact Fairies will have on Gira's usage. And besides, if you're leaving Gira in against Fairies you know can beat it, you deserve whatever you get.
And finally, where did you get the idea that there's no offensive ground, water, and fire resists in the tier? Keldeo resists Fire and Water, while Lando-I is immune to ground. Mega Altaria can also be used offensively and it resists Fire and Water, and is immune to Ground before Mega Evolving. So, three out of the four S-ranks, all three of which see play on offensive teams, resist or are immune to AT LEAST one of those.
EDIT: I forgot to put Griseous Orb on my original calcs. After scolding people for underestimating it. Why do I insist on doing this stuff late at night? I've corrected my original calcs. And, now that I have Griseous Orb on:
252+ SpA Griseous Orb Giratina-O Draco Meteor vs. 248 HP / 108 SpD Mandibuzz: 246-291 (58.1 - 68.7%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
Now, Mandi's still a great counter, but that does make a pretty big difference.