Just wanted to point out that your set can't win since your pokémon HAS to switch in the opponent, meaning if the opponent leads with abra you MUST take an attack on the switch-in AND take another attack on the turn where you can actually do something. Shroomish can't win since Psychic 2HKOes on the switch
Shroomish does have an 11% chance of surviving 2 Abra Psychics, and that chance rises to 90% if it is Poisoned. I already covered all of the scenarios in my post, and I did admit that Shroomish by itself is not the most reliable way to counter the core in general but probably one of the best single mons that can do the job well enough. Like I also said, slapping on any Abra counter and you'll have a solid anti-core, but then it won't just be one Pokemon doing all of the work, will it?