Genesect is one of those pokemon that look real good on paper but I guess are just average during actual battles. The team that I'm using right now looks pretty weak to Genesect, as each one of them can be OHKO'd by a +1 SE move. However, the reason why Genesect isn't fit for Ubers is because it relies so much on choiced items to work effectively. Genesect really just isn't fast enough to pull off an effective offensive set that poses pressure against opposing teams, so it usually carries a scarf to remedy that problem. Then, it gets the speed it needs, but it loses its ability to truly use its wide coverage- and although it is a great U-Turner it is worn down very quickly by Stealth Rock. I've seen RP sets, yes, but it pretty hard for Genesect to set up enough boosts to sweep without it being crippled or KO'd. And although it gets Download, it's still kind of weak, and definitely can not be compared with the behemoths such as SpecsTios or Bandorus.
I'd say the real Trick with dealing with Genesect is to a.) get SR up early and b.) know its moveset. Stealth Rock is pretty vital to countering Genesect, because it switches in and out an average of 3-4 times each match. Hazards really wear Genesect down because of how much it likes to pivot- which, to add to how to play WITH Genesect, means that spinners are really important if you plan on revenging or sweeping with Genesect. Still, however, most spinners don't exactly have good synergy with Genesect. Due to its for defenses when uninvested, nearly any moderately powerful STAB neutral move can deal heavy damage. Foretress can spin for Genesect, but it just compounds on its mighty x4 weakness to fire, and if you add a fure resistant pokemon like Heatran, other problems will start popping up. Donphan is in a similar position- water type moves will screw them both over, especially Scald because Genesect is crippled heavily by burns. Starmie and Genesect show a weakness to Electric, and because Volt Switch/Thunder are the most common electric moves in the tier, it won't be hard to either switch to gain momentum or just flat out KO one of them.
Hazards are a really big problem, and if you can get those down, it'll be a lot easier. Another important thing is to know Genesect's set. It might sound trivial to say that you'd treat a pokemon's scarf set different from how you'd treat a pokemon's boosting set. However, this should be even more emphasized for Genesect, which really relies on those crucial SE hits in order to deal real damage. This makes the scarf set a bit predictable; and though experienced players will think things through and try to come up with a good play, Genesect's scarf set will always suffer from prediction-itis. It's RP set is hugely different, however. For one, special walls all become much more effective- due to Genesect's lack of a physical move, Blissey, Chansey, and SpDef Jirachi can all tank hits and reply accordingly. However, with the RP, prediction is no longer as useful, because getting Lando-T in on a Tbolt won't matter- they'll just outspeed amd KO you with Ice Beam. Against a scarf set, however, you'd want to be able to predict a Tbolt, and then switch into Lando to apply revenging pressure. It's important to know Genesect's set, because the differences are so extreme that a mess up could cost you a match.
Offensively, Genesect players should try to cover those weaknesses as much as possible. Because spinners are really important, you'll often have to put one in anyway, and then let the rest of your team patch up. Because it us easy to play against Genesect when you know its set, Genesect players should try to be mysterious- make some unpredictable plays, feign choice, or just plain outpredict. Don't forget to apply offensive pressure, and not let your opponent get those opportunities to have an advantage over your ancient bug.