Moves
Scald's most important function is fishing for burns, which lets Seismitoad not be entry hazard fodder for Skarmory and Ferrothorn, since they hate being burned on the switch. Refresh is what makes Seismitoad so valuable, since negating Toxic, Toxic Spikes, and Will-O-Wisp allows Seismitoad to neutralize bulky Water-types like Rotom-W, Politoed, Tentacruel, Milotic, and defensive Starmie. The primary use of Protect is to stack Leftovers recovery and sand damage, but it also lets Seismitoad scout Choice-locked Pokemon like Latios, Tyranitar, and Garchomp. Stealth Rock is an amazing option, as Seismitoad can pressure all spinners with the combination of Scald + Refresh and free up a moveslot for its teammates. Alternatively, Knock Off can make progress by removing items, making foes like Ferrothorn, Refresh Politoed, and opposing Seismitoad unable to regain health with Leftovers. Earthquake can be used to damage other Water-types—especially Keldeo, break Kyurem's Substitute, OHKO Heatran if it's not maximum HP, and damage Toxicroak. However, it lacks any other important targets, since Seismitoad has mediocre offensive stats, and forces it to run a Relaxed nature. Toxic can also be used in the last moveslot to deal with other Water-immune Pokemon, like Gastrodon and Jellicent, and to not be overly dependent on Scald burns to make progress. Dropping Protect for another support move is possible but never recommended, since losing the stability it provides alongside Leftovers hurts Seismitoad a lot in the long run.
Set Details
28 Speed EVs are used to outspeed standard specially defensive Heatran. 248 HP and 232 Defense EVs with a Bold nature prevent Seismitoad from being 2HKOed by Jolly Sand Force Excadrill's Earthquake after Protect. If running Earthquake, use 248 HP / 252 Def / 8 SpD with a Relaxed nature.
Usage Tips
Do not waste any opportunity to bring Seismitoad safely to the field, since almost every team has a bulky Water-type. Spam Scald against Pokemon like Ferrothorn, Skarmory, Jirachi, and Tentacruel, as burns are the main way for Seismitoad to threaten them. Use Refresh as soon as Seismitoad gets inflicted with status so to not waste any HP. Do not let common Substitute users go unpunished, as Seismitoad attracts them, oftentimes being unable to break the Substitute of Pokemon like Breloom and Keldeo. Going for Scald to burn the foe or outright switching to achieve an advantageous position is best in these scenarios. Avoid using Protect too predictably. If using Stealth Rock, bring Seismitoad in as soon as possible to set it up. If using Knock Off, do not be afraid to predict switches, since its best targets in Ferrothorn, Latios, Jellicent, and Reuniclus love to switch into Seismitoad. Refresh, Stealth Rock, and Knock Off are Seismitoad's advantages over Gastrodon, so it should use them to their fullest.
Team Options
Since Seismitoad loves the chip damage sand provides, Tyranitar is one of its best partners, and it also Pursuit traps problematic Psychic-types like Latios and Reuniclus. Pokemon that can deal with Ferrothorn like Heatran, Breloom, Magnezone, and Xatu are a must, since Ferrothorn can use Seismitoad as entry hazard fodder. Xatu especially is an amazing partner for an anti-metagame core, shutting down entry hazard stacking and helping Seismitoad with Reflect support. Magnezone invalidates any free switch in that Ferrothorn or Skarmory tries to grab versus Seismitoad. Because Seismitoad is a bit passive, it loves having Spikes support from Pokemon like Ferrothorn and Skarmory. Offensive Pokemon that can benefit from Seismitoad's ability to nullify Rotom-W like Excadrill, Mamoswine, and Garchomp are also amazing partners. Pokemon that have ways to deal with Reuniclus like Latios, Scizor, Encore Alakazam, and Nasty Plot Celebi are needed on the team. Bulky Flying-types like Landorus-T, Gliscor, and Skarmory can help deal with Adamant Excadrill. Seismitoad can be used in rain structures to neuter Rotom-W, which is normally a nightmare for rain teams. In those, partners like Gyarados, Tentacruel, Scizor, Breloom, Latios, Latias, and Jirachi are good options.