Gengar isn't winning vs Goodra, Mega Gengar is KOed by Infestation / Rest Shuckle outside of a Sandstorm. AV Tyranitar easily wins vs Gengar, tanking the Focus Blast as a 3HKO. Ferrothorn threatens T-Wave. Talonflame revenge kills. Aegislash revenge kills. Scizor pursuits. Scarf pokemon revenge kill easily (Scarf Chomp Earthquake is fun). I'm not saying that Mega Gengar isn't a threat... but it certainly hasn't blown holes in my team. I can think of very many Sp. Def walls that serve their job as wall, and still deal with nearly all permutations of Gengar that I can think of.
O.k im going to break this post down, because it contains a few inaccuracies. Firstly
Gengar isn't winning vs Goodra, Mega Gengar is KOed by Infestation / Rest Shuckle outside of a Sandstorm. AV Tyranitar easily wins vs Gengar, tanking the Focus Blast as a 3HKO
Gengar has easy access to Taunt, which shuts down Shuckle (assuming Mega Gengar actually WANTS to trap and kill Shuckle), not to mention I suspect Perish Trap Gengar still wins regardless. Goodra ill give you, but thats not really the point, Mega Gengar is ONLY going to be switching in on things it can trap and kill, and then its going to switch out again and do it all again. You have a Skarmory walling my Dragonite? Thats fine ill send in Mega Gengar and take that shit down, switch out, and then try taking something else down later on. This is what is frustrating about Shadow Tag users, because there is very little counterplay to them, and thus, calling Goodra a counter isn't entirely accurate, since Gengar simply won't stay in (or switch in), on something it won't beat, rather it will come in when you are in a vulnerable position, abuse your inability to switch, trap, kill, rinse and repeat. The only viable defence against this is a Pursuit user, which are few and far between, and Gengar can win against most of them. Tyranitar for instance, can Pursuit Gengar all it likes, but Gengar has the option of using Destiny Bond, and taking you down with it, in addition to the pokemon it just trap killed for a 1 for 2 trade, Scizor, if Banded, risks losing to Disable variants, and its easiy to scout for BP / Pursuit via Protect techniques.
Ferrothorn threatens T-Wave. Talonflame revenge kills. Aegislash revenge kills. Scizor pursuits. Scarf pokemon revenge kill easily (Scarf Chomp Earthquake is fun).
Ferrothorn hates taking a Focus Blast / Taunt / Substitute into Perish Song / Disable variants etc etc, Scizor still needs to worry about disable, Talonflame and Aegislash might revenge kill, but Gengar isn't going to stay in anyway after its trapped and killed something, and Scarfed revenge killers lose to Protect ---> switch to resist / immunity. You don't have the option of a double switch, because Shadow Tag blocks it, always giving you the upper hand. In addition, all of those lose if Gengar managed to come in, and trap something that does nothing to it, thus gaining a free sub and rendering your attempts to revenge kill it largely useless.
I can think of very many Sp. Def walls that serve their job as wall, and still deal with nearly all permutations of Gengar that I can think of.
Feel free to name them, because I don't know many that win against Sub Perish, or even Taunt versions.