Shuckle should be the Hazard Setter
On the SS Monotype Viability Rankings, Shuckle is S Rank while Armaldo is not on the VR, or Unranked. That should already indicate which one is better. The reason why Shuckle is the better hazard setter is because of it's access to both Sticky Web and Stealth Rock. Rock-Mono usually uses Offensive HO, so Sticky Web is important so Rock-type teams can outspeed threats such as Jirachi while Sticky Web is up. Here is the Shuckle set used on Rock Mono:
Shuckle @ Mental Herb
Ability: Sturdy
EVs: 248 HP / 252 Def / 8 SpD
Bold Nature
- Stealth Rock
- Sticky Web
- Knock Off
- Encore
Knock Off is used to remove items from Pokemon trying to switch in to Shuckle, and Encore locks Pokemon like Hydreigon into using a move like Nasty Plot, who can take advantage of Shuckle. Mental Herb is used to bypass a turn of Taunt so you can get Sticky Web up.
If you still need a Rapid Spin Pokemon, I'd recommend Coalossal, as it can check Pokemon like Ferrothorn and also has access to Spikes. Here's the Coalossal set if you need a Rapid Spin Pokemon:
Coalossal @ Heavy-Duty Boots
Ability: Flame Body
EVs: 252 HP / 252 Def / 4 SpD
Bold Nature
IVs: 0 Atk
- Flamethrower
- Rapid Spin
- Scorching Sands
- Spikes
TL;DR Shuckle is used as the hazard setter since it has Stealth Rock and Sticky Web, with Sticky Web being useful for Rock Monotype teams. Armaldo is not a good Rapid Spin Pokemon, as Coalossal is the better Rapid Spin Pokemon for Rock-type teams.
Hope this helps! :)
