I came up with a set for Gengar not listed in the OP. Given Gengar's immunities to Fighting and Normal, it is simple to set up Calm Minds if you run into Choiced users of those moves. If you get 6 Calm Minds and a Substitute in, good game.
Keep Calm and Sweep:
Gengar
Ability: Levitate->Shadow Tag
Item: Gengarite
Nature: Modest/Timid
EVs: 4 HP / 252 SpA / 252 Spe
-Substitute
-Calm Mind
-Shadow Ball
-Dazzling Gleam
Gengar gets Dazzling Gleam, which basically completes its Ghost/Fairy coverage. While it's not as strong as Sludge Bomb or Focus Blast, it lets Gengar not be completely walled by Chesnaught (which I still find funny), and it would let Mega Gengar basically kill all Choiced or mono-attacking Fighting or Normal moves. I remember using a similar set with Chandelure in Dream World, but Mega Gengar does it better. Come in on a Choiced Normal or Fighting attack, set up a free sub and 6 free Calm Minds. You get perfect coverage except for Pyroar (which is 2HKOed by a +5 Dazzling Gleam if it runs 252/252+, otherwise, less boosts will work.), and you literally 2HKO every unboosted Pokemon in existence except Blissey (98.83% chance to 2HKO at +6 with Modest after SR!) and Chansey at +6. You also OHKO Shuckle after SR in Sand, you destroy Tyranitar, and you OHKO Arceus-Normal after SR. Every other Arceus form is taken down similarly easily.
It's not perfect, since faster Infiltrator mons can stop it, and breaking its Substitute and then killing it (requires 2 faster mons, kinda hard) works too, but if used in the correct conditions, it can and WILL sweep entire teams, regardless of which tier it ends up in.