As far as I see, Genosect has a really good shot at it using a Rock polish set. If you ever thought AgilLucario can stop offensive teams by speed-sweeping them, take a look at Genosect! Solely, this set:
Genosekuto @ Aqua Cassette
Trait: Download
EVs: 252 SAtk / 4 SDef / 252 Spd
Modest Nature (+SAtk, -Atk)
- Rock Polish
- Bug Buzz
- Techno Buster
- Thunderbolt/Ice Beam
You'll need to time the switch perfectly in order to pull off the Rock Polish. If you manage to pull off a +1 special attack boost through download by switching in correctly, you can consider chances of yourself outright winning the game! If this Genosect ever gets a +2 speed and +1 special attack, it is a real force!
Now, how this set works after the boost is something to look at. Firstly, we have a good STAB move in Bug Buzz which will hurt quite a lot considering the power it comes off of.
The notable Bug Buzz resisters are Skarmory, Gyarados, Salamence/Dragonite, Heatran, Blaziken, Shandera and Burugeru among many many others. The other two moves are just for this reason. Thunderbolt and Ice Beam is a really tough choice. If Dragons are walling you so hard, Ice Beam can go well with it, destroying them with OHKOes and also hitting the many Flying types which resist Bug Buzz. If water types are giving you hassle, Thunderbolt it is. Note that the overall coverage of thunderbolt in this set is much greater than ice beam, so that is something to consider too.
Oh, the Techno Buster is rather essential here. A water attack here is very nice to take out those Fire types that will wall you hard especially if you are going with Ice Beam. Seeing how effortlessly Heatran and Shandera stop Genosect, a water type Techno Buster is very useful. If you go with Thunderbolt, there is no reason to believe that water Techno Buster is not that effective. It gets the many Ground types out there who will otherwise wall you.
It is tempting to use The BoltBeam here and sacrifice water Techno Buster for one of these moves as well as another item, but there are so many offensive teams out there who get really messed with a fast water type attack user.
Looking at what this genosect stops, we can see many offensive Pokemon getting out-sped right in their tracks and OHKOed. Blaziken can't outspeed +2 speed Genosect even if the Protect works 2 times in a row and Techno Buster is an OHKO. Scarf Shandera is a common switch-in, who is also OHKOed by this and out-sped. Heatran is another one which makes it on the KOed list (albeit this one will probably need the +1 special attack boost to OHKO). Sazandora, Tyranitar and many other Darkies are OHKOed by STAB. There are many more out there, but I'll look at the one I am most concerned about: Doryuuzu. +2 Modest Genosect out-speeds Adamant Doryuuzu in Sandstorm and OHKO with Techno Buster, but Jolly Doryuuzu, though less common, isn't out-sped. So this might be one reason to consider a Timid nature for Genosect, though I don't think Jolly Doryuuzu is all that common. It is something to keep in mind.
As for counters, this version of Genosect (like every other special version) is stopped cold by Blissey. I bet you saw that coming, but that's why we have 5 other Pokemon in a team. Nattorei stops him too, but can only threaten to cripple with Thunder wave. If thunder wave is absent and Genosect has pulled off a +1 special attack download, Bug buzz will hurt and Leech seed just isn't enough to survive the power, especially since Nattorei cannot even touch Genosect with direct moves.
Finally, a surprising counter I found was Ninetales. Though this set manages to take Ninetales somewhat well, he can be a great stop to almost any Genosect. A 100 base speed to guarantee out-speeding and coupled with 100 base special defense to take those shots helps switch in well. Water techno buster is greatly weakened by the Drought ability and Ninetales can just OHKO with a fire move. As I have been seeing, Thunderbolt seems to be the ONLY consistent move on Genosect's sets that can at least hit Ninetales neutrally. Something to think about as a Genosect counter, if not at least a check. Oh and a +1 Hidden power ground from a non-Life Orb Genosect (even from Modest) doesn't OHKO 0 special defense 0 HP Ninetales.