This reminds me of a cool RMT from a year ago that uses a variation of this set, actually, in that case used to trap special walls more broadly for Specs Kyurem. I've tried to play around with some variant of the concept, and I think it's so good in theory, where your opponent reacts to an incoming Volcanion by bringing in Glowking, Mola, or some other bulky water like Pex, but I think in practice it faces the issue of opponents reacting to the Volc by sending in their dragon instead (Dnite/Pult/Kyurem/Bolt/some other nicher options like Hydreigon or Latios). Basically, if your opponent doesn't send in something passive to take what they presume is an incoming Steam Eruption, then now you're conceding a free turn to something very scary and getting very little out of it besides like 20% of chip. And of course, once you've revealed that you're running Fire Spin with no SpA investment, then your opponent knows that they don't have to respect the Specs Steam Eruption anymore and the slot becomes a liability (even with bulk this thing does not check anything).After some more testing of my mixed Manaphy set, I have come to the conclusion that it is not bad. It is also not particularly good. The problem is the opportunity cost of using something better. Water is an ok attacking type. It's just that there are a lot of resists and immunities for a set using like no coverage moves. I tried to build around this in several ways for my teams, but everything I tried seemed to be more trouble than it was worth. At this point, the only thing I haven't tried is rain brute force. However, I think it is safe to say that Rain has better breakers. Also, the special Manaphy sets are better breakers. I think I am done with this experiment. If Wellspring goes, it might be worth revisiting. Otherwise, there are better options.
However, I did find a rather interesting anti-meta mon in my quest to remove bulky Water types:
Volcanion @ Air Balloon
Ability: Water Absorb
Tera Type: Poison
EVs: 248 HP / 252 Def / 8 SpD
Bold Nature
IVs: 0 Atk
- Fire Spin
- Taunt
- Steam Eruption
- Sludge Wave
I originally made this with the intent to remove Waterpon and bulky Water types that got in Manaphy's way. Tera Poison Sludge Wave was primarily for Wellspring. However, it has several other uses like resisting Body Press or status proc. You also hit most Water types neutrally.
Fire Spin allows you to trap Water Absorb mons. I'd say this makes it a better trapper than Heatran since you have Steam Eruption to hit the rare Fire immunities. And thanks to your own Water Absorb, Flip Turn mons like Alomomola are completely trapped. In fact, it's really hilarious just how hard this things bodies Alomomola strats. Even Fire Spin itself can be used to counter Mirror Coat.
Aside from Wellspring, I found that going full defensive was helpful in dealing with many of the threats. If you have high HP, you can live a hit from Great Tusk and KO with Steam Eruption even without the Air Balloon. With Air Balloon, you can threaten quite a few Ground types.
Finally, the combination of Steam Eruption and Sludge Wave is surprisingly good at spreading status against slower or more passive teams.
With that said, I do remember that this thing murders stall, especially if you run Tera Ghost, so I wonder if you could build a team that combines Fire Spin Volcanion + special breaker with four slots designed more specifically for anti-offence. Something like a five-mon stall core with a trapper that imitates those Dug/Gothitelle stalls from earlier gens, or pairing it with double priority +setup Dnite, something along those lines.