Yes, I think you're all wrong. Gengar ex CAN shine thanks to Rare Candy. And it's all thanks to Grandma.
Running 2 Candy + 1 Stage 1 would allow us to evolve three Basic Pokemon, theoretically. And we can run three of the same Stage 2 to evolve into thanks to ex and non-ex variants being different cards. But we can't run more than two of the same Basic. Except we can, thanks to Celestic Elder!
Game plan is super simple, and readily apparent at the first glance of the list. Candy ASAP into Bother Gengar, build a second Gengar, and after the first one is inevitably knocked out, bring it back from the grave to become GENGAR EX. Spooky Shot is not so weak now if they need to get 4 points out of you to win!
The choice of Supporters is an interesting one. Guzma can capitalize on Gengar's effect by also locking them out of Tools while yours are protected. Lillie makes it impossible for them to keep up with just Potions and Malasada's available to them. There are cases to be made for switching any of these out for Mars and/or PokeCenter Lady, but I think they aren't necessary. Mars is unnecessary, since they are locked out of playing their Supporters anyway, and the game will go too long for them not to have drawn their evolutions for her to disrupt much of anything. Guzma is better too, as it guarantees that the Tools that they played stay out. And PCL's Status heal may be potentially helpful, but both Statuses aren't too common, and Lillie's heal of 60 is bound to compensate for the loss in the off-chance they show up. It's better to have a more consistent heal of 60.
A 2-1 split of Poison Barb and Giant Cape is there so it'll be more likely you can Poison their attacker before the last Gengar comes in and can rack the Poison damage further with Cape. Malasada is not nearly as common as PokeCenter Lady, so they will be forced to switch around to weaker Pokemon if they intend to cure it. Barb is preferred over Helmet since it allows for the damage to keep racking up after one Gengar equipped with it has been knocked out, potentially totalling 30-40 damage instead of just 20. Coupled with one hit of Bother and Spooky shot, it adds up to 180-190 damage, enough to kill all other exs after Guzma has blown away their own Capes.