SkarmBliss is not invincible.
Garchomp, Gyarados, Salamence, Tyranitar are not invincible.
Weavile, Heracross and Infernape leads are not unbeatable.
Everyone seems to use them, no doubt thinking they are the best in the game, and I know some people can't stand fighting them.
SkarmBliss is ruined now basically, since Skarmory can pe pwnt straight through his physical defences anyway and the physical/special split means Skarm isn't so hard to beat down.
Neither is Blissey, Close Combat fixes that little problem.
Garchomp, Salamence, Gyarados and Tyranitar are all standard pokemon since they have pretty good stats and a decent standard moveset to put on them, but they aren't invincible. A Boltbeamer with some decent defences can shoot down Garchomp, Salamence and Gyarados - Porygon2 for example, with Ice Beam, Thunderbolt and Recover would work quite nicely, if you wanted to make him a novelty pokemon, breed for HP Fighting and counter Tyanitar at the same time too. >_>
Weavile, Heracross and Infernape - the 3 most common leads (followed by Metagross and Breloom from what I can gather) all have crap all defences, any pokemon with a decent defence stat and nice attack or special attack will counter them right off the bat. A Kingler lead could counter them all at the same time, with Rock Slide.. Crabhammer should go on him too, since it can't get Waterfall.