Amoonguss A- -> B+
I really have not seen this mon at all in the last two months. In the playoffs for SPL 8, for example, it had exactly zero usage. I can't speak as to its overall usage in March or April because afaik the stats aren't up, but in February it was hovering around the 70th most used (at 1695+ play; overall it was much lower, of course... something like 140th), which is lower than all the rest of the A- mons bar Mew (which has gone from B- to A- in the last month and shot up in usage with Medichamite's release) and Medicham (medichamite was unavailable so). But usage alone isn't enough, though it does indicate that people are shying away from the mon for whatever reason.
Speaking of Medicham, it eats Amoonguss for breakfast. In fact, almost every relevant offensive threat in the tier OHKOS or 2HKOs Amoonguss - Greninja, Lando, Autotomize Celesteela, Lele, Volc, Zygarde (fully invested, but SubCoil sets up on it so either way), Chomp, (Specs) Gengar, Heatran, Mawile, Offensive Scizor, M-Alakazam (and regular for that matter), CharY/X, (Specs, after rocks) Magnezone, Marowak, Medi, Mega Pinsir, and (Zen headbutt) Bulu. These are just from the A and S tiers.
The offensive mons I didn't include from A are Koko, Ash-Gren, Magearna, and Keldeo. But if you wanted a defensive answer for these, why not use AV Tangrowth? Tangrowth has better bulk on both sides, and in a meta full of Landorus, Lele, and Zygarde, arguably a better defensive typing. Yeah, amoonguss checks Magearna and Keldeo (and dazzling gleam Koko) a bit better due to its typing, but that's pretty much it.
Amoonguss can't fulfill its role as a defensive pivot if it can't consistently come in on meta threats. Sure, you could get it in via double switches volt turn and such, but there's so many better options to do that with (all the insanely strong wallbreakers hanging around). Its big selling point over Tang, Spore, is useless if it never gets a chance to use it. It just doesn't seem near as consistent a defensive mon as the 4 stall mons and Mew in A-, and the increased psychic usage from Lele and Medi (and Gren too) really doesn't help it. Nor does it like all the hazards going around, as spikes and stealth rock undercut its bulk even more and add some KOs to its list (2HKOs for A-Gren, Magearna for example). Grass/Poison really just isn't as effective a typing as it once was, as evidenced by Venu's drop from A- to B-. And obviously, there's always Koko (a bit more popular with medicham kicking around) and Fini, whose terrains ruin its day.
Finally, Taunt has recently become a much more popular option for many mons in the effort to combat stall, among other things. Amoonguss *hates* taunt.
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In summary: It can't come in on much in the current meta, and is threatened by a lot. Therefore it can't use spore near as much as it would want to. A secondary Poison typing is kind of a hindrance compared to Tangrowth with the ground and psychic types all about. Psychics have gained a stronger foothold in the meta after Meta's ban both as offensive wallbreakers and defensive answers to those wallbreakers. Increased hazards suck for it, and taunt's increased usage really sucks for it.
I don't think it's belonged in A- for a long time, but the recent increase of psychics has really cemented that belief for me.
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That said, B+ is a good place for it. While it finds difficulty switching in, finding a switch in to it is actually pretty difficult. Not much that can deal with spore/stun spore wants to stay in on it, and spore is just really awful to deal with if you don't have terrain support and/or Tangrowth/some other grass not 4x weak to HP Fire. It also provides a nice middle ground between Toxapex and Tang as a regen user in terms of utility and offensive presence, if inferior to both in a straight up comparison. Being in the same tier as Fini makes sense - both are good defensive mons who are affected by unfavorable meta conditions.