Amoonguss can stay in A-. While it has awesome bulk, killer ability, and is a solid switch in to Hitmonlee and almost every water type ever (questionable because Slowking is the most common but w/e) it has quite a few flaws that don't make it A worthy in my opinion.
First of all it is not even the main choice as a Grass type in stall as it can't take advantage of the free turns it gets bar Spore while Roselia can set up (Toxic) Spikes and act as a cleric and, with Sleep Clause active, it gives a free switch to threatening Pokemon like Roost Moltres and any Steel-type setup sweeper (Durant, Doublade) who can't really do the same with other "passive" walls like Alomomola who at least can threaten a Scald Burn.
Gligar deserves a bump to A. It is easily the best defogger in the tier as it can switch in and use the move with surprising ease (after Omastar is dead Gligar usually has 1-2 switch in opportunities against Hazards offense) for balanced teams and a blanket switch to most physical attackers (including the all so threatening Hitmonlee) that usually need multiple boosts to break it (unless you are Durant or SubBU Braviary) giving it time to Status them or 2-3HKO if they are Poison or Steel types immune to Toxic. It can also "spread" Knock Off pretty easily, gain momentum with U-Turn, use Taunt, and setup Stealth Rock if you are desperate.
Might edit in more in-depth reasoning later but I think that B is very cluttered (it had like 20 mons) and these are the mons I can see going down:
Cofagrigus, Gastrodon, Heliolisk, Slurpuff, Kabutops (for this I'm not sure because I haven't used it that much), Tangrowth.
First of all it is not even the main choice as a Grass type in stall as it can't take advantage of the free turns it gets bar Spore while Roselia can set up (Toxic) Spikes and act as a cleric and, with Sleep Clause active, it gives a free switch to threatening Pokemon like Roost Moltres and any Steel-type setup sweeper (Durant, Doublade) who can't really do the same with other "passive" walls like Alomomola who at least can threaten a Scald Burn.
Gligar deserves a bump to A. It is easily the best defogger in the tier as it can switch in and use the move with surprising ease (after Omastar is dead Gligar usually has 1-2 switch in opportunities against Hazards offense) for balanced teams and a blanket switch to most physical attackers (including the all so threatening Hitmonlee) that usually need multiple boosts to break it (unless you are Durant or SubBU Braviary) giving it time to Status them or 2-3HKO if they are Poison or Steel types immune to Toxic. It can also "spread" Knock Off pretty easily, gain momentum with U-Turn, use Taunt, and setup Stealth Rock if you are desperate.
Might edit in more in-depth reasoning later but I think that B is very cluttered (it had like 20 mons) and these are the mons I can see going down:
Cofagrigus, Gastrodon, Heliolisk, Slurpuff, Kabutops (for this I'm not sure because I haven't used it that much), Tangrowth.