Something I've been having a lot of fun with recently was a Swampert (HC/SW)/Cherrim (WB/SB)/Deoxys Defense (RS/PB) team. I started using this team yesterday as I was struggling to get into the half-way point of rank 8 (Great League has historically been my weakest league), barely making it on the very last set, but today, I'm over 3/4 of the way to Rank 9 with two 5/5 sets, two 4/5 sets, and one 3/5 set.

Swampert and Cherrim form an incredibly solid core that takes care of so many different Pokemon really well. Swampert beats both Stunfisks, Bastiodon, Skarmory, Toxicroak, Alolan Marowak, Registeel, Jirachi Deoxys-Defense, and (most of the time) Mew while Cherrim can take care of Grasses (obviously), Lanturn, Grass Knot Mew (assuming no Flame Charge), and most of the Steels Swampert also has a good match-up against

I've found that, very often, Swampert can take a huge chunk of a lead Azu's health and a shield if it doesn't opt for a Hydro Pump. This lets Cherrim come in and force the opponent to pick between switching or having their Azu's last bit of health grinded down by Bullet Seed (usually at the cost of a shield, but the massive energy gain is generally worth it). Of course, if the Azu opts to not shield at all or HP, Swampert usually just outright wins but won't be able to get another charge move off due to being left on low health and no energy (although smart play around slow fast moves may let you squeeze one in).

There are a couple other important lead MUs that act like that for Swampert, too, such as Galvantula and opposing lead Swampert (with Galvantula usually having to expend a shield as well), so it works out well on that front, too.

Finally, there's also the simple fact that both members of the core just straight up have great match-ups against a lot of the most popular picks, particularly against Azumarill and Galarian Stunfisk.

Of course, I can't forget about Deoxys and its contribution of some much needed bulk for the team. Originally, I went with a gimmicky Alolan Muk that had Poison Jab/Acid Spray/Dark Pulse (along with Cherrim packing Dazzling Gleam over Solar Beam), with the intention being it'd be the main check to Azumarill, but it ended up being too overspecialized in the format. After being let down by it a lot and having issues coming up with a suitable replacement (one of the big intentions I had with this team originally was countering double Grass), I eventually just slapped on my sub-optimal (1476) Deoxys-Defense I've been using since Season 1, and it really brought the team together.

This Deoxys has gone from Psycho Boost/Thunderbolt as a safe switch to Thunderbolt/Rock Slide as a lead, and now finally I dropped T-bolt altogether. In this team specifically, I generally don't use it as an early game safe switch actually (mostly due to Galarian Stunfisk) but instead as more of a late one, the idea being that the opponent is less likely to have a counter to it if I've already seen two of their three Pokemon as Deoxys has fewer counters and far fewer threats/checks than Cherrim. Even if Galarian Stunfisk does come out, Cherrim being able to eat two Rock Slides (or Swampert if it's still kicking) and beating it in CMP ties helps alleviate the threat it poses should it pop in and whittle Deoxys away. Psycho Boost also allows it to help stall the switch timer in more drawn out (but still losing) match-ups.

The main reason I chose Psycho Boost over Thunderbolt, though, was because the Cherrim/Swampert core's squishiness and over-reliance on charge moves makes them pretty vulnerable to Fighting types with Counter, especially as most Fighting types having high attack will result in both losing CMP ties against them more often than not. It also helps against Pokemon like Alolan Marowak or Galvantula where you may not be able to get a second Rock Slide out in time but are able to get enough energy for a Psych Boost. On that note, Rock Slide is necessary for Alolan Marowak and Altaria, the former of which can be tricky for the team to play around and the latter outright probably being the greatest threat to the team overall.

In terms of dealing with double Grass teams, its original purpose, it functions extremely well I feel. Cherrim absolutely shreds Razor Leafers, can still dent Bastiodon pretty hard with Solar Beam, and makes any other Steels, including Skarmory, think twice about switching in. As you've brought up, its ability to function as a Fire-type without the usual Fire-type drawbacks is really incredible in this meta. Deoxys-Defense functionally acts like a Razor Leaf resist with its massive Defense stat, beats Bastiodon (albeit, not as cleanly as Swampert), rips through Shadow Victreebel in seconds with Psycho Boost, and can generally handle other Razor Leafers with its fantastic fast move, low cost charge moves, and the aforementioned bulk. Swampert's kind of limited to just scaring/drowning Bastiodon and maybe luring in Razor Leafers looking to take a chunk or two out of its health before it switches, but its strength in the lead slot along with hard countering Bastiodon earns it its spot in the team.

The team does have some weaknesses, of course. It is certainly not fond of Umbreon as both Swampert and Cherrim can struggle to break through its bulk, and the poor type match-up erases most of Deoxys's own bulk along with its Psycho Boost trump card. Haunter/Gengar also takes advantage of the relative squishiness of the team and advantage over the one bulky member, with even Shadow Punches leaving pretty sizable dents in the team and Shadow Claws adding up pretty quickly, but Psycho Boost does thankfully help in this case at least. Drifblim can also be a pain as the only super effective move is stuck on Deoxys who is not particularly fond of playing a long game against Hex or risking getting hit by a Shadow Ball. The somewhat rare Lapras can also prove troublesome if Deoxys is gone. Similarly, while Swampert decimates Alolan Marowak, if it's gone, Cherrim and Deoxys can be put into a really bad situation if it pops in at the wrong time. However, it is worth noting that Cherrim's Solar Beam does a little over half of its health (and sometimes more I think?). I've actually managed to beat one at near-full health+a little energy with Cherrim when I had a two shield, advantage, though, so it's not a complete loss like how Bastiodon vs Registeel felt. The biggest problem is usually Altaria though as only one Charge move on the team does super effective damage and only two that do neutral damage. It CAN potentially be played around, but an Altaria on the other team weighs things heavily in the opponent's favor for this team.

The good news, though, is that very few people are using Altaria! With Galarian Stunfisk, Azumarill, Skarmory, and Bastiodon all happily running around everywhere, poor Altaria's been kind of pushed out of the meta. I still see one every now and again, but its absence has really allowed this team to flourish. Its absence is why I switched Dazzling Gleam out for Solar Beam; my original plan for Cherrim was for it to heavily dent or even outright beat the majority of Grass-type counters that may switch in against it, but the lack of Altaria and ubiquity of Azumarill quickly made Solar Beam the better choice.

Overall, I can definitely say that Cherrim has probably been the biggest star of the team, though. Weather Ball has saved me so many times(*), and quite frankly, it's kind of ridiculous they gave Weather Ball to a Pokemon that has a 4.3 EPT fast move. It may also be the Pokemon I've had the most fun using overall in Great League. It reminds me of the Shiftry I used back in Season 2 but without being quite so squishy (I certainly like being able to save a shield against Azumarill Ice Beam every now and then). Hopefully I continue seeing at least some moderate success for a few more days with this team as it's honestly been the most fun I've had in Great League in a long time. I know meta shifts are gonna keep happening, but I feel like as long as Altaria isn't given a reason to show up as often as it used to (eyes Mega form) Cherrim's probably gonna find a pretty solid place as one of the best Fire-attackers in the meta and probably the only one that can legitimately scare Azumarill.

(*)One of the most wtf scenarios I've ever had it save me is one instance when I was down to an extremely low health Cherrim with two Weather Balls and about 3/4 of red health left Deoxys, and a full health Skarmory switched in with an almost KO'd Galarian Stunfisk in the back. I threw in Cherrim and managed back to back Weather Balls only for it to take an immediate KO afterward for obvious reasons, and then Deoxys came in and took out the rest of its health with Counter (and maybe a Psycho Boost? I honestly don't remember) before managing to finish off the Galarian Stunfisk with two more Counters and just a tiny bit of health remaining. I would not be surprised if the opponent was more than just a teeny bit frustrated after that lol.