In Monotype, there are a limited viable Pokemon generally for any type. It's not really hard to pick out a last member—just open up the viability rankings and from the few remaining 2-3 choices pick one. Moving on, let's go to the rate.
Rillaboom
The set you're using is the standard set and the EVs are the general purpose ones, so there's no issue with the moves or EVs whatsoever. However, Adamant Nature is the preferred choice for Rillaboom. The Spe isn't a big deal honestly, and you outspeed faster Pokemon with a priority Grassy Glide anyway so having the higher attack is much more important.
You also have the option of using a Bulk Up Zarude set instead of Choice Band, which is equally good. The choice is yours, if you go Bulk Up then Rillaboom should be Choice Banded, else SD Rillaboom works nicely with Banded Zarude. Here's the set for preference:
> Rillaboom @ Choice Band
Ability: Grassy Surge
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Grassy Glide
- Wood Hammer
- Superpower
- High Horsepower
Kartana --> Rotom-Mow
Rotom-Mow @ Choice Scarf
Ability: Levitate
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Thunderbolt / Nasty Plot
- Leaf Storm
- Volt Switch
- Trick / Defog
Unfortunately for our "Paper Plane", it was so broken that the Monotype council had no choice except for banning it from SS Monotype. Therefore, we have to replace it for another Scarfer so why not Rotom-Mow?
Rotom-Mow is probably one of the best Scarfer you can use. Along with that, it has incredible utility with moves like Trick Scarf + Nasty Plot, Defog as well as forming a VoltTurn core with Rillaboom. Trick cripples walls, giving the perfect opportunity to use Nasty Plot. However, Defog can clear Stealth Rock, Sticky Web, etc. and notably vs Bug monotypes because of Levitate.
Amoonguss is perfectly fine, but Iapapa Berry is a bad option; Black Sludge gives guaranteed recovery over long term or even Rocky Helmet is annoying for physical attackers.
I think I'll quote this from the analysis:
Choice Band significantly increases Zarude's damage output, giving it the means of 2HKOing Toxapex with Power Whip and specially defensive Celesteela with Darkest Lariat. However, Zarude then misses out on the ability to remove status conditions like burn and Toxic poison using Jungle Healing, which leaves it more easily crippled.
Source
Zarude @ Leftovers
Ability: Leaf Guard
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Bulk Up
- Darkest Lariat
- Jungle Healing
- Taunt / Drain Punch
You're better up using a Bulk Up set which is a bulky sweeper, and we can certainly task Rillaboom with a Choice Band. It gets Taunt to deal with Toxapex or prevent Ferrothorn's spikes, two which are very annoying for Grass-monotype since Rillaboom is choice banded. Darkest Lariat is helpful vs Iron Defense Corviknight / Ferrothorn, and has no immunities. Bulk Up allows it with ample set-up opportunities, while Drain Punch can surprise Steel-types like Heatran and is a niche but useful choice.
Ferrothorn
Ferrothorn @ Rocky Helmet / Leftovers
Ability: Iron Barbs
EVs: 252 HP / 4 Atk / 252 Def
Relaxed Nature
IVs: 0 Spe
- Spikes
- Leech Seed
- Gyro Ball / Iron Head
- Protect / Body Press
The set you're using is weird. Leech Seed is like a must-have, without the key recovery Ferrothorn won't have much longer survival. Grass-monotypes generally rely on Cradily as their Stealth Rock setter over Ferrothorn, whose job is racking up Spikes. Iron Defense is also a move that can't fit here—you need Spikes as well as Steel-STAB + Body Press for Clefable and preventing free switch-ins to Heatran.
If your only attacking move is Body Press, then any Calm Mind user like Tapu Fini or Clefable can abuse it. Since it's a grass-monotype, we focus on hitting Clefable, not Tapu Fini. Iron Head has more PP, and is more useful in longer battles vs Pressure Kyurm. Leftovers go well with Protect, giving a free turn of recovery.
And the final Pokemon of the team is gonna be....
Cradily!
Cradily @ Leftovers
Ability: Storm Drain
EVs: 252 HP / 4 Atk / 252 SpD
Careful Nature
- Rock Blast
- Toxic
- Stealth Rock
- Recover
We sure do a need a specially defensive wall, as well as missing Stealth Rock, which is one key to pressure Fire and Flying monotypes' constant switching and Cradily does that. Being neutral to Fire and Flying thanks to the Rock-type, it's a valuable addition because of the sheer utility it has.
Rock Blast is useful vs the same Pressure Kyurem mentioned, it also helps against Substitute Galarian-Moltres and Hydreigon which try to use it as a set-up bait. Rillaboom's Knock Off helps to get Heavy-Duty Boots off, allowing Cradily to punish with constant Stealth Rock chip.
Final Team: https://pokepast.es/2f83c6f6da335069
Hope this helps! :)
