I also dislike how much people are downplaying how much momentum Genesect generates. "KO or free switch" is so powerful with offensive teams as the amount of offensive pressure you can keep, should your opponent switch, with other fast pokemon like Greninja, Analytic Starmie, Terrakion, Landorus-I, Keldeo, Thundurus-I and Latios, all of which are strong and fast in their own right. Hard hitters and wall/stall breakers like of Sub-3 attacks Kyu-B, Mega-Pinsir, Azumarill and Bisharp also appreciate free switches on things they threaten with their power. Not to mention Life Orb Deo-S and more notably, Mega-Lucario.
Genesect has a fast U-turn and many of the aforementioned partners are frail and can be dealt by anticipating their switch-in and attacking them. This requires something to sponge a U-turn. If Genesect scores a kill, it has to forfeit momentum. Scarf Genesect creates parity due to it being choice-locked against teams that are not immensely weak to its common attacks.
Genesect's below average bulk for a pivot prevents it from switching in, and critically, reliably going Scarfless to sponge hits from faster Pokemon, and switch out. It
almost always has to use
fast U-turn because cannot switch in on anything or absorb hits despite its good defensive typing, and that is an inherent limitation that one can play around.
The "weakness" lies in the loss of momentum after a KO (this is already a net gain on your part), which can be patched up with offensive pivots like Landorus-T, Aegislash, AV Conkeldurr and Rotom-W.
Gliscor shut all those Pokemon down due to its Volt Switch immunity, U-turn resistance (it takes chip damage from it), and passable bulk. At best, prediction from AV Conkeldurr removes the Toxic Orb from the switch and Hydro Pump does not guarantee a KO when Stealth Rock is off the field, and often the core's Stealth Rock setter is Lando-T and it is hapless from Gliscor, whose best option is to U-turn away from the core into the periphery before it can set up Stealth Rock. Ice Punch without a status doesn't KO Gliscor, although Life Orb + Iron Fist does.
I do not know the calcs, but one can put special defensive investment to ensure it survives uninvested Hydro Pump at full health or probably after Stealth Rock. Survival is more important than the amount of health to Gliscor since it has the Ground and Electric immunity and good speed to spread Toxic if it decides to sacrifices itself.
An unorthodox option to deal with Gliscor within the core can be Knock Off instead of U-Turn on Lando-T to hit Gliscor on the switch, and is highly superior to U-Turn in that case, but this is highly situational. Manectric cannot OHKO with HP Ice.
In general, Gliscor does not mind the entry hazards that the Genesect user can leverage.
Genesect gives that core a chance against Gliscor. Only a chance, those as Genesect has to forfeit momentum just to break the Sub or kill it with prediction on the switch. Genesect can kill Gliscor but it does not counter it.
The narrow distribution of Ice Beam and shittyness of HP Ice this Gen 6 help Gliscor a lot. It also shutdown the common spinner Excadrill when its Balloon is popped and status and outspeed Mandibuzz for status. Thundurus-I needs Life Orb to kill it and does not get free switches. Icicle Spear or Cloyster on the physical side? Also, Kyu-B needs a clean field to destroy things and has many counters.
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I apologize for the disquisition into Gliscor but one can inquiry into how Gensect synergizes cores, especially against its problems...
As I pointed out with the Gliscor example,
Genesect can only be as good as its partners that it can U-turn out to, and that's if it is being used as a U-turner, not revenge killer. It was banned last gen due to its capacity to be a terrifying sweeper,
in addition to the roles it can perform as a Scarf and EBelter.
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Gliscor also loses to any special attacker, and anything with Substitute. Especially Gengar. Or Kyurem-B if you can get it in without getting Toxic'd, Substitute Kyurem beats just about every staller in the game.
But this thread is for the suspects, not Gliscor, so let's get back to that.
I slapped on a Kyurem-B with a Rotom-W, Lando-T, and AV Conkeldurr core. It obviously needs a "free" switch. Kyurem-B can handle Stealth Rock even with Life Orb, but Toxic would be lethal. Even if it is in, it would only force Gliscor out into a check. At best it would force Gliscor out. Kyurem-B is too ponderous and does not like to switch out; I intend to use it on a stall-breaking run when its checks has been removed.
Kyurem-B cannot beat Blissey or any Fairy.
Gengar can get a free switch in the SubToxic set but it needs to watch out for Knock Off.
Genesect is only as a good as its partners. It can been seen as part of a core, or the periphery of the core. I do not primarily regard Genesect as a U-turner as I do not highly value its defensive traits or see much defensive synergy with Rotom-W or Lando-T, so I do not see it as part of a Volt-Turn core with Rotom-W. I am more likely to use Hydro Pump prophylatically than risk the Volt-Switch being predicted.
Its efficacy also depends on the state of the field. This is a corollary to the adage that "Genesect is only as a good as its partners".
It does not solve any of the problems that the defensive core it supplements suffers, but it certainly has the tools to remedy them. That is why Genesect is an asset,
but the effective application of those tools can be thwarted by the opponent and the core would be rendered impotent. I do not use hazard removal so I have less control over hazards. If Genesect is not supplemented by hazard control, its efficacy wanes dramatically. As a result, I cannot use it liberally as a momentum generator, and I have to live with that. Hell, even the odd Galvantula Sticky Web put it at a disadvantage.