Stall is a playstyle that really got nerfed in BW2.But I feel it is still viable and not impossible.Sure NP Thundurus-T,Mixed Hydreigon,SD Haxorus in rain are wall breakers that almost no defensive core can wall.But these mons are rarely used.I agree with alexwolf that "Stall is impossible this meta" is bullshit.It just requires some variation.Especially using revenge killers like previously mentioned.
Genesect perhaps played a vital role in nerfing stall,but I don't find it "extremely" hard to counter at all.There are many viable defensive cores out there that can stop Scarf Genesect,Pink Blobs and Heatran hard counters RP and Ebelt variants(unless it has hp ground but it hampers it's coverage moves).CB Genesect is something which I agree is major threat to stall with it's ability to beat both physical and special walls thanks to download and wide movepool.But Heatran still checks it.
One fact is overlooked when considering stall is that a lot sets and mons which BW2 brought lacks in usage.Because stuff like CB genesect and Mixed Hydreigon are of little use against offensive teams which are far more popular.
The pokemons which are nicely viable overall and is really threatening to stall are Taunt Tornadus-T ,specs Keldeo,Growth Giga Drain Venusaur and such.Xatu is something remotely dangerous to stall imo and it was popularised by lavos sun.Really,how are you going to beat Xatu when it will bounce any hazard or status.It also has nice bulk,reliable recovery and decent attacking options.
BW2 also gave some notable gifts to stall too.Amoongus is a bitch to face.Walling major threats like Breloom,Keldeo and Thundy-T to an extent is really impressive.Add the ability to heal just by switching is amazing.Not to mention access to spore to shut down something as well as ability to spread paralysis.I had nice success with a stall team featuring HAS core in early BW2.There were also other things like Gliscor getting Poison Heal+Roost,Ferrothorn getting Leech seed,Spikes,SR in one set etc.These are not to be ignored.