Marshtomp's great defensive typing makes it a great wall and counter to Electabuzz. Gabite does compete in these roles, and it may appear to outclass Marshtomp with its better Speed and even better defensive typing. However, Marshtomp beating Ice Punch Electabuzz and checking Fire-types better makes it plenty worthwhile. With RestTalk, it can afford to take repeated hits and chip damage. It only needs Earthquake as an attack because, while it cannot harm most Flying- and Grass-types, using Stealth Rock when they switch in suffices as punishment. In the event the team can afford another Stealth Rock user, then Marshtomp could opt for Scald or Yawn instead to pressure foes with status, though neither are as effective as Stealth Rock. A mixed defensive EV spread works best, as Marshtomp does not need much special bulk to counter Electabuzz, whereas extra physical bulk helps with Raboot.
Defoggers and Rapid Spin users like Vullaby and Wartortle often switch into Marshtomp when it uses Stealth Rock. Switching to an offensive teammate like Electabuzz, Choice Band Raboot, and Fraxure as they clear Stealth Rock is a great way to bring them in safely and preserve momentum; if they don't go for removal, then Marshtomp at least secured Stealth Rock to punish them later on. As for Hattrem and its Magic Bounce, use Dark-types like Galarian Linoone, Vullaby, and Zweilous to beat it, as well as offensive partners like Lampent and Thwackey that can switch into Giga Drain with no problem. Defensive partners that resist Grass are also a must; physical walls like Roselia, Tangela, Ferroseed, and Koffing complement Marshtomp in this vein by checking Thwackey.
Thanks to an excellent defensive typing, Marshtomp excels as a special wall that checks Fire-types such as Lampent and specially offensive Combusken, blocks pivoting attempts from Electabuzz and Klang, and threatens foes with Earthquake. Toxic cripples common switch-ins such as Thwackey and Tangela. Alternatively, Yawn can be used to force switches and synergize with Stealth Rock to prevent Defoggers from removing entry hazards. It also punishes Ferroseed and Duosion, which are both immune to Toxic, from switching in. Marshtomp can also opt for a mixed spread of 252 HP / 152 Def / 104 SpD and an Impish nature to better handle Piloswine and Electabuzz, while still avoiding a 3HKO from Electabuzz.
Marshtomp should aim to use its typing to force switches and set up Stealth Rock early-game and then focus on walling special attackers. Physical walls such as Koffing, Tangela, and Gurdurr benefit from Marshtomp checking special wallbreakers such as Gastly and Lampent; in return, they handle physical wallbreakers such as Thwackey and Gurdurr. Pokemon that provide entry hazard control like Gurdurr, Hattrem, and Vullaby keep Marshtomp from being quickly worn down by residual damage. As a wall that lacks reliable recovery, Marshtomp greatly appreciates support from Wish and Healing Wish users such as Clefairy and Hattrem. Pokemon that can check Grass-types such as Golbat, Roselia, and Koffing are solid partners, as they cover for Marshtomp's glaring 4x weakness.
Moves
Stealth Rock is essential for every team and Marshtomp is a good user of the move, as it forces a lot of Pokemon out
Scald is chosen over Waterfall for its burn chance, letting Marshtomp wear down checks such as Quilladin
Earthquake hits most neutral targets for solid damage and has good coverage with Scald
Ice Punch is used to deal good damage to Grass and Dragon-types
Toxic is Marshtomp's only way past opposing defensive Porygon2 and other bulky threats
Set Details
Maximum Defense investment makes Marshtomp as physically bulky as possible
A Relaxed Nature is used to preserve Scald's power, as Marshtomp does not need its Speed
Usage Tips
Pivot Marshtomp into the Pokemon it checks, such as Rhydon and Monferno
Scout for lures such as Freeze-Dry Piloswine or Hidden Power Grass Electabuzz that can hamper Marshtomp's ability to check what it needs to check
Team Options
Grass-type checks such as Sliggoo and Fletchinder are good partners for Marshtomp to help with his 4x weakness to Grass. Sliggoo can even check Electric-types that may carry Grass coverage so Marshtomp doesn't have as much pressure put on it
Wish users such as Togetic can synergize well with Marshtomp, as it can pass Marshtomp Wishes to increase its longetivity
Pokemon that like a good switch-in to physical attackers, like Monferno and Electabuzz, appreciate Marshtomp's capabilities to check these threats