The team looks to be taking a bulky hazard stacking approach with residual damage from sand and Reuni to abuse the passive damage, so it's time to start looking at Goltres checks. The most splashable one in the current meta fits nicely here:
Nihilego @ Black Sludge
Ability: Beast Boost
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
- Grass Knot / Thunder Wave / Toxic / Stealth Rock / Toxic Spikes
- Power Gem
- Sludge Bomb
- Knock Off
Nihilego provides a reliable Goltres switch-in, made even more reliable by the SpDef boost from sand. Crucially, it also accentuates the passive damage doled out by this team thanks to Knock Off. Since Hippowdon is already carrying Stealth Rocks, they aren't needed on Nihi, and Grass Knot can be used instead to punish Swampert and Excadrill. Other options could be Thunder Wave to rack up free turns, Toxic to add to passive damage and hit non-Steel Flying types, or again, Stealth Rock to keep hazards up more reliably in the face of Defoggers like Salamence and Mandibuzz. Toxic Spikes could also be used to punish Hippowdon, Swampert, and Chansey over time, rather than needing to hit them directly.