I've used Gastrodon a lot, specially at the beginning of BW2, but things have changed quite a bit so any input would be nice. Main changes in the current analysis would be stop mentioning Quagsire.
Gastrodon
[OVERVIEW]
<p>Gastrodon is a Pokemon blessed in typing and ability, boasting two immunities, four resistances, and a lone double weakness to Grass-type attacks. As if that wasn't enough, fifth generation buffed Storm Drain by granting it a Special Attack boost. Not much has changed in the transition to BW2, where Gastrodon wasn't used much due to its relative frailness and weakness to status,; however, the ban of Genesect and Tornadus-T has given some breath to our beloved sea slug, being able to be part of giving it a place in anti-rain and rain teams without the constant fear of being obliterated.</p>
<p>Gastrodon is able to be used in rain, sandstorm, or weatherless teams thanks to its attributes, and with its useful ability it comes in handy in the current Water-type plagued metagame. Gastrodon can completely wall common Pokemon as Starmie and Rotom-W with ease, being immune or resistant to their main attacks. But not everything shines for it, for Celebi and Ferrothorn and other counters are pretty common in the Overused metagame and its bulk is not as great as it may seem, making easy to take advantage of Gastrodon, either easily KOing it or setting up.</p>
[SET]
name: Defensive
move 1: Earthquake / Earth Power
move 2: Scald / Ice Beam
move 3: Toxic
move 4: Recover
item: Leftovers
ability: Storm Drain
nature: Sassy / Calm
evs: 252 HP / 84 Def / 172 SpD
[SET COMMENTS]
<p>This set enhances Gastrodon's special bulk, allowing for an offensive and defensive presence at the same time. Despite the lack foof Defense, the suggested EV spread allows Gastrodon to survive a CB Close Combat from Terrakion and a Choice Band Outrage from Garchomp at full HP. It will also sponge both Psyshock and Secret Sword, taking 56.33 to 66.43% from Choice Specs Latios and 39.67 to 47.18% from Choice Scarf Keldeo. While Earth Power benefits from a Storm Drain boost and Gastrodon has a marginally better Special Attack stat, unboosted Gastrodon will usually find Earthquake better to deal with threats with a high Special Defense, such as Jirachi and Bulky Swords Dance Scizor.</p> (new paragraph)
<p>Gastrodon can be used effectively in any kind of team,; in sandstorm teams it won't take the passive damage, but it shines more in a rain team, since Scald damage is boosted and it becomes a more perilous threat. It can check and counter common offensive threats for both rain and sandstorm teams, being able to absorb all incoming Hydro Pumps, Scalds, Thunderbolts, and Thunders that would otherwise seriously damage your team members. With its bulk it can take some special hits, force a switch, and then Recover back the health. It can even take the uncommon Starmie's Grass Knot and two Draco Meteors in a row from Life Orb Latios, forcing it to switch and healing damage back. In a rain team, it can take whatever Tyranitar tries to hit it with and answer with a Scald, which, as we all know, will most likely burn it and thus render it useless. In fact, Scald's burn chance is usually a better option than Ice Beam, which you can use to deal with many of the Dragon-type and Ground-type Overused threats, but Gastrodon can't really take many neutral physical hits, so a Scald burn on a switch-in will help it do its job. Gastrodon can spread status easily with its ability to force switches, so Toxic is a must, especially to deal with foes s as Rotom-W and Starmie, that get walled but don't take much damage from Gastrodon's puny attacks. There is no discussion when it comes to Recovery, any defensive Gastrodon should use it and there's no reason not to. Recover is not up for discussion; every defensive Gastrodon should use it.</p>
[ADDITIONAL COMMENTS]
<p>You can use a more specially oriented spread of 252 HP / 4 Def / 252 SpD to make Gastrodon able to tank Thundurus-T and Rock Polish Landorus hits better. An EV spread of 252 HP / 192 Def / 64 SpD with a Bold nature is also possible and will make Gastrodon able take physical attacks such as Terrakion's Close Combat. With this spread, Gastrodon has more chances to use Ice Beam effectively switching into a physical attacker or trying to burn Terrakion or Scizor with Scald, since it will be able to take a hit or two. Keep in mind, though, that even on maximum Defense, Technician Scizor's U-turn still deals a lot of damage. It's best to use Recover against a Scizor predicting it to heal back to almost full health. (I didn't quite understand the last sentence... consider rewording? predicting it to what?) Be aware that despite all its traits, Gastrodon isn't really bulky enough to tank and defeat continuous powerful attacks, and that any status will ruin it and render it useless.</p>
<p>The best teammates for Gastrodon are, ironically, its foes,; Grass-types will resist attacks thrown to at Gastrodon and might be able to answer with a coverage move. Ferrothorn is a good ally, since it can take most of the Grass-types, even some Hidden Power Fire under the rain while the rain allows it to handle even some Hidden Power Fires, and put some Spikes in the field. Latios and Latias will take any Grass-type attack too, additionally as well as resisting Fighting-type attacks and obliterating Fighting-types with ease. In a rain team, SubToxic Tentacruel is another good teammate, since it can take Jellicent, though Scizor is recommended to Pursuit it,; additionally, it handles Grass-type attacks well too. Tornadus-I, useful on rain teams due to its powerful Hurricane, can take Fighting- and Grass-types as well.</p>
[SET]
name: Special Attacker
move 1: Surf / Scald
move 2: Earth Power
move 3: Ice Beam
move 4: Hidden Power Grass / Recover
item: Choice Specs / Life Orb
ability: Storm Drain
nature: Modest
evs: 124 HP / 252 SpA / 132 SpD
[SET COMMENTS]
<p>The suggested spread allows Gastrodon to be an offensive presence, dealing several (did you mean severe? consider changing it altogether, as you use 'severely' a little further on) damage with Surf or Scald, Earth Power, and Ice Beam, and it is still able to take some hits, just like such as one Draco Meteor from Choice Specs Latios. Using Choice Specs, under rain, and adding in a boost from Storm Drain, Gastrodon's neutral coverage makes it an offensive presence to be dreaded, as it can severely hurt a lot of opposing Pokemon, even 2HKOing Blissey with Surf. Maximum power Surf and Earth Power can 2HKO Ferrothorn if you predict the switch-in. Most opponents will expect defensive Gastrodon,; use this to your advantage, surprising and surprise unsuspecting foes dealing high damage with a with a high-damage Surf or an Earth Power. With Choice Specs, under rain and with a Storm Drain boost, Gastrodon 2HKOs Blissey with Surf. Choosing the moves, yYou can use Scald over Surf because it is that damn good: it will burn physical opponents often, granting you a huge advantage, and burn damage will add up against special attacking opposing Pokemon. Hidden Power Grass obliterates opposing Gastrodons, even defensive variants, and deal with Rotom-W and Politoed. You could use Recover on a forced switch to get health back if you are using Choice Specs, but usually Hidden Power Grass is a better option. A Life Orb set can be used with Recovery, but you miss out on 2HKOing Ferrothorn, and Gastrodon really appreciates the extra power on from Specs; but switching the ability to switch attacks is very welcome though. Life Orb set will use a spread of 104 HP / 252 SpA / 152 SpD to get the least amount of recoil from Life Orb, meaning you can attack from full health once more. Keep in mind when using an offensively oriented Gastrodon that, despite its possible raw potential power, it's still very slow and won't be able to tank many hits before fainting.</p>
[ADDITIONAL COMMENTS]
<p>You might use 124 HP / 252 SpA / 132 Spe with Choice Specs or 104 HP / 252 SpA / 20 SpD / 132 Spe with Life Orb spreads to outspeed and 2HKO Chansey after Stealth Rock damage 47.3% of the times, and Blissey after Stealth Rock and Spikes damages or two switch-ins; under this assumes rain and with a Storm Drain boost. If you are using Life Orb, you could run Rain Dance in the fourth moveslot to help with weather wars in forced switches,; Hidden Power Grass is recommended though to deal with Water-types or Recover to heal back the recoil. Keep always Always keep in mind that even though Scizor is crippled by Scald, it outspeeds and can use U-turn. Don't be brave, and; switch out of it; unless you can Scald it on the switch-in.</p>
<p>Offensive Gastrodon will appreciate the same kind of Pokemon that its defensive counterpart does,—mainly Grass-type resisting and status absorbing teammates, also some as well as Pokemon able to take the common hard-hitting attacks like Hurricane, Close Combat, or and Outrage. Ferrothorn is the perfect teammate for any Gastrodon, as it can take an incoming Toxic, Grass-type attack, U-turn, or and Dragon-type attacks. Heatran covers Gastrodon's weakness perfectly as (space) well and is a good offensive presence at the same time, being able to deal with dragons.</p>
[OTHER OPTIONS]
<p>There aren't many other viable options, really. All of them are mainly outclassed. For instance, you'd rather use Toxic over Yawn when inflicting status (explain, maybe? just a mention that yawn is unreliable, idk~). You could choose Clear Smog as a second move to stop a set up, but its damage will be very low. Memento as a last move could come in handy, forcing a switch when Gastrodon is about to faint anyways/s]. Gastrodon can run too Counter and Mirror Coat to deal back huge damage to opponents, but generally the other options will work better. Stockpile can be used, but it is not recommended on in the current hyper offensive metagame, as most of the times you will be wasting turns using it or you will be letting opponents to set up on you. You could run Hidden Power Fire, but do not bother with it, Scald does better against Scizor and Ferrothorn will OHKO Gastrodon with Power Whip. Additionally, you can use Sand Force in a sandstorm team for a more powerful Earth Power, albeit losing Gastrodon's main niche and one immunity. Sand Force Specs Earth Power will tear appart opposing Steel-types, bar Skarmory. However, Storm Drain will be a better choice most of the times, since Gastrodon welcomes a lot its spot as Rain countera lot of Gastrodon's effectiveness lies in its use as a rain counter.</p>
[CHECKS AND COUNTERS]
<p>The main bane of Gastrodon are Grass-types: Venusaur, Breloom, Celebi, Roserade, Amoonguss, Virizon, Ferrothorn, and others like them are an immediate switch out for our sea slug friend. Physical attackers, especially Ferrothorn, must be wary of an incoming Scald, since the burn can ruin them., although Ferrothorn will most likely switch-in anyways, Leech Seed Gastrodon, and procceed proceed to set up all the hazards possible. Most Natural Cure users can take a Toxic and switch out and even take hits from unboosted Gastrodon,—Blissey, Chansey, and Shaymin will have no trouble switching into him it. Hydration Vaporeon takes little damage from it and will be able to get rid of Toxic too. Despite being uncommon, be aware that Politoed and Heatran could can use Hidden Power Grass to dismiss Gastrodon in one turn. Fast Substitute and Taunt users will tear Gastrodon apart (redundant) into shreds too,—Gyarados, Latias, Jellicent, Substitute + Dragon Dance Dragonite, and Substitue Kyurem get a free switch-in on Gastrodon and a free set up turn. Gastrodon is weak to status itself and Toxic will cripple its utility, forcing it to switch out before having its HP devoured. In addition to all the previous threats, Gastrodon will have a hard time facing powerful physical attackers such as Choice Band Dragonite, Terrakion, Kyurem-B, Grass Knot Thundurus-T, Keldeo's Secret Sword Secret Sword Keldeo, Salamence, and Garchomp;, even though at full health or with some bulk it can survive a hit from these.</p>