Moves
Scald is used as Gastrodon's main STAB move, as it can burn opposing Pokemon and enable Gastrodon to wall powerful physical wallbreakers like Dragonite and Mega Gallade. Earth Power is Gastrodon's secondary STAB move and synergizes well with Storm Drain, letting it always 2HKO Toxapex after a boost. Recover is Gastrodon's form of reliable recovery, which lets it act as an efficient wall. Toxic is used to cripple most defensive Pokemon like Rotom-Wash and Porygon2 that are not bothered by Gastrodon's STAB moves.
Set Details
Storm Drain gives Gastrodon an immunity to Water-type moves and is the main reason why it's considered as a valuable teammate for Ground. It also has the side effect of raising Gastrodon's Special Attack stat when hit by a Water-type move, making both Scald and Earth Power hit slightly harder. Maximum HP and Defense investment with a Bold nature makes Gastrodon as physically bulky as possible and enables it to never be 2HKOed by Choice Specs Keldeo. Leftovers gives Gastrodon passive recovery throughout the match and is also needed to beat Choice Specs Keldeo.
Usage Tips
Gastrodon is best used when switched in on predicted Water-type moves coming from potent threats like Choice Specs Keldeo, especially if no other viable moves from those Pokemon can seriously harm it. Since Gastrodon is the team's Water-immune Pokemon, it should not be sacrificed, because that would enable threatening Water-types to greatly damage and KO most of its teammates with their STAB moves. Gastrodon also has the luxury of switching into would-be threats for Ground teams, such as Rotom-Wash and Tapu Koko, without much repercussion. However, watch out for lures that could cripple or OHKO Gastrodon, such as Toxic Rotom-Wash and Grass Knot Tapu Koko. Scald can be used freely to burn, cripple, and wear down physical attackers as they switch into Gastrodon or try to overpower it. Burning defensive Pokemon that don't care about Toxic, such as Mega Sableye and Celesteela, to slowly whittle them down is also a good idea. Use Toxic to cripple defensive Pokemon like Zapdos and Mantine on the switch or to put setup sweepers like Kommo-o and Mega Slowbro on a timer. Against hard matchups, specifically Water, spreading Toxic early-game can also greatly help to give Ground the advantage. Also, when facing Pokemon that are unable to 2HKO or poison Gastrodon, it is advised to Toxic these foes and stall them out with Recover so that Gastrodon has as much HP as possible to check the remaining threats. Try to keep Gastrodon healthy by using Recover so that it can take, if possible, two hits from opposing threats. If this condition is met, Scald or Toxic can be used depending on the remaining Pokemon on the opposing team. However, since it drains momentum from its own team, Gastrodon should be played carefully and make good use of double switches by sponging a Water-type move or a weak attack before switching out to a more favorable teammate if threatening foes like Kyurem-B and Celebi are still alive. Moreover, as Gastrodon possesses very little offensive presence, stallbreakers like Mew and Hydreigon greatly threaten it with Taunt. To play around them, try to double switch as they come in or switch out to a more appropriate teammate on a predicted Taunt to gain momentum, since Gastrodon has no way of dealing with those Pokemon by itself. Finally, like most defensive Pokemon, Gastrodon's stalling capabilities become greatly limited when it's hit by Toxic. Therefore, it's better to avoid opposing status users unless they can't be dealt with another way, such as Porygon2, than by using Toxic against them.
Team Options
Every Ground team appreciates having a teammate such as Gastrodon that can switch into Water-type moves. Hippowdon and Gastrodon form a nice defensive core, as the former walls strong physical attackers like Mega Lopunny and phazes setup sweepers like Mega Scizor, while the latter can switch into Greninja's and Keldeo's moves with ease. Mega Garchomp is a remarkable teammate, as it's the most viable Ground-type (aside from Seismitoad) that outspeeds Azumarill and survives its +6 Aqua Jet after Stealth Rock damage. It can also help Gastrodon deal with Ferrothorn and other Grass-types with its Fire-type coverage moves. Excadrill is a much-needed spinner on Ground teams to get rid of entry hazards, especially Toxic Spikes, which seriously hinder Gastrodon's longevity and ability to wall certain threats. Excadrill can also clean through weakened teams more easily if paired with Gastrodon thanks to its ability to cripple its switch-ins with Toxic. Additionally, Excadrill can take on most relevant Grass-types for Gastrodon, since they take serious amounts of damage or are OHKOed by its STAB moves. Landorus is a useful and powerful wallbreaker that helps Gastrodon take care of Steel- and Grass-types such as Celesteela (with Gravity support) and Mega Venusaur. Mamoswine is an alternative wallbreaker that eases the Grass and Flying matchups with its Ice STAB moves, being able to pick off weakened Tapu Bulu and Breloom with Ice Shard and take out other threats, namely Zapdos and Ferrothorn, with Icicle Crash and Superpower. Mamoswine also has the added benefit of bringing a secondary Ice-type switch-in thanks to its ability Thick Fat. Mega Steelix is a more niche teammate that performs incredibly well in certain matchups, namely Fairy and Dragon, that Gastrodon greatly struggles against. Gliscor can also from a defensive core with Gastrodon thanks to its Grass-type neutrality and ability to efficiently stallbreak. In fact, Gliscor can shut down many of Gastrodon's defensive answers, such as Celesteela and Mega Veusaur, with Taunt before slowly wearing them down with Earthquake or Knock Off, and, thanks to Poison Heal, can switch into opposing Toxics aimed at Gastrodon. In return, Gastrodon can switch into Ice- and Water-type moves aimed at Gliscor. Dugtrio is a nice teammate that traps and helps Gastrodon dispose of numerous foes like Chansey, Kyurem-B, and Tapu Bulu.