I don't see any good reason to demote Amoonguss. When you look at the top threats right now in OU, no need to be a genius to realise how good Amoonguess can be.
- If you want you counter Keldeo without being Pursuited (I mean, just that point makes the pokemon worthy).
- You can also check effectively Terrakion, there is no coverage move that can kill you. So basically, if it's a choiced version, you almost took 0 damages with Regenerator, and won the tempo.
- You wall Breloom. All day, all night.
- You got Spore... SPORE. Do I even need to explain how strong this thing can be ? "Let's get free momentum, and a free "OHKO" with a defensive pokemon, life is so easy"
- Can go mixed. Both his defense and his special defense are decent enough to be able to build a set. I mean, if you want to have a physical wall, go ahead, if you want to have a special wall to take some HPump, go ahead. My favorite set is a physical one with enough investment in SpD to take 2 HPump from Specs Politoed in Rain.. and it does not require too much investment, I can even make sure that I won't be 2HKO by Stone Edge from Terrakion. A physical wall able to take 2 Hpump under rain .. O.O
- It has Regenerator, and this thing is damn strong.
I laughed at "Psychic Landorus", I hope it's not your only reason to demote Amoonguss. And seriously, Alakazam is far from being enough to justify a demote.
Now, about what can set-up on the shroom. I think it's your fault, from your building, if something sets-up too easily on him. He has 85 base SpA .. This is really "OK" for a defensive mon. You got HP Ice if you're afraid of Dragons, Para Spore if afraid of.. anything.. and Clear Smog aswell.. I don't really like Giga Drain by the way. I just think that you should really should get specific coverage for specific threats when you play Amoonguss and then you won't be disapointed.
I think B rank is really fine, and it's not because one can't have success with a pokemon that it justifies a demote IMHO.
PS : This thing has SPORE.