Moves
This set aims to act as a solid utility counter and defensive pivot, taking advantage of Swampert's excellent typing and bulk. It is capable of taking on powerful Choice Band users like Tyranitar, Infernape, and Dragonite. In addition, it does very well versus Jirachi, Bronzong, and Metagross. Sadly, its lack of a reliable recovery move significantly hampers its tanking abilities.
Swampert can set up Stealth Rock well due to its bulk and powerful Earthquake to keep spinners like Starmie in check. With high Defense and resistance to popular attacking types like Rock and Fire, it can set up entry hazards under many circumstances. Earthquake is Swampert's best STAB move, hitting anything that doesn't resist it for a great portion of damage. Ice Beam is used in the third move slot to enable Swampert to OHKO Flygon, Gliscor, and Dragonite after Stealth Rock damage. It also allows it to hit Grass-type switch-ins, such as Breloom, for super effective damage, though Swampert must switch out on the next turn. Roar is the primary option in the final moveslot, as it can be used to scout the opposing team and rack up residual damage from Stealth Rock. Protect can be used as a scouting move and can also aid Swampert in blocking Explosion from Metagross and Azelf leads and giving Swampert an extra turn to restore its health with Leftovers.
As far as other options go, Swampert has a few choices. Stone Edge can be used in place of Ice Beam to dish out respectable damage to Gyarados and Dragonite. Hydro Pump and Surf can be used if you'd like a secondary STAB option for Swampert to use, which can be useful for wearing down foes like Skarmory and Bronzong, though you'd have worse coverage.
Set Details
240 HP EVs maximize Swampert's Leftovers recovery and minimize damage from entry hazards and status. Also, the HP investment with Leftovers allows Swampert to always survive Seismic Toss from Clefable and Blissey five times at full health and gives it more all-around bulk. 216 Defense EVs grant Swampert exceptional physical bulk, letting it take most resisted and neutral physical attacks with ease. The remaining Special Defense EVs are used to allow Swampert to take neutral and resisted special attacks, such as Dragonite's Draco Meteor and Heatran's Fire Blast, more easily. An alternate EV spread of 252 HP / 252 Def / 4 SpA can be used to maximize Swampert's physical bulk, but the extra Defense EVs don't accomplish anything specific, and the Special Defense investment is generally more useful.
Usage Tips
Swampert should be used as a defensive wall and entry hazard shuffler early-game. Swampert acts as a decent lead, as it can put Stealth Rock up reasonably well against popular leads such as Zapdos, Metagross, Heatran, and Hippowdon and frail Choice Scarf leads such as Infernape and Flygon, and it can usually dominate most offensive Pokemon when it has the type advantage. Since Swampert does not have a reliable recovery move, it's important to bring it into Rock-, Fire-, Electric-, and Steel-type attacks that it resists. Swampert finds plenty of opportunities to come in thanks to its Stealth Rock resistance and sand immunity, which is great for balance teams. Also, Swampert's Attack is high enough naturally to use Earthquake as a way to pressure opposing Starmie, which helps maintain Stealth Rock and forces it to use Recover, which can give Tyranitar or Rotom-A an opportunity to switch in without having to sustain damage from a Water-type attack.
Team Options
Swampert should be used on semi-stall and balance cores, as these are where its defensive abilities shine the most. Since Swampert can be successfully used on multiple types of teams, it has many viable teammates.
On balance cores, Pokemon such as Scizor and Heatran can be used to compensate for its 4x weakness to Grass-type moves. Both can take on Celebi and Shaymin, two of the biggest threats to Swampert, though they must watch out for Hidden Power Fire and Earth Power. Mixed Dragonite and Infernape can take advantage of the Stealth Rock that Swampert sets up by forcing the opponent to switch through offensive pressure, which hurts the opposing team and gives your team offensive momentum. Another solid way to play Swampert is to utilize a Fire / Water / Grass core with Pokemon such as Heatran and Roserade. This will force the opponent to switch multiple times due to defensive synergy, racking up more damage from entry hazards. On the other hand, Sleep Talk users such as Choice item Latias and Choice Band Dragonite also pair well with Swampert, as they are able to switch into Grass-type moves with ease and absorb Breloom's Spore and Roserade's Sleep Powder.
On more defensive teams, specially defensive Jirachi and Clefable make wonderful teammates for Swampert, because they can sponge special attacks that could otherwise spell doom for the mudfish Pokemon. They can also pass Wish to Swampert to keep it healthy, which can be a tremendous blessing because Swampert has no instant recovery move. Tyranitar is an excellent choice to consider thanks to its ability to shut down popular threats that appreciate taking advantage of this Swampert such as defensive Rotom-A, Starmie, Choice Specs Latias, and Celebi and punish them with its powerful STAB-boosted Dark-type attacks. In addition, it can summon sand with its Sand Stream, which helps rack up residual damage. On the other hand, since Swampert appreciates entry hazard stacking, a core with Skarmory and Rotom-A with Tyranitar in the back to trap Starmie makes for a solid defensive balance.
Physically defensive Swampert tends to struggle against stall teams because it does not hit hard enough to actually threaten Pokemon like Skarmory, Rotom-A, and Clefable. The aforementioned Pokemon can take advantage of its passivity to set up Spikes, status it, or remove its Leftovers with Knock Off. Therefore, playing Swampert alongside stallbreakers can help mitigate this weakness. Breloom makes for a great partner, as it can form a solid Fire / Water / Grass core, if also paired with Heatran, and it can take advantage of slow bulky Pokemon like Clefable and defensive Rotom-A. As mentioned, Heatran also makes a decent partner, due to its appropriate typing synergy, but also for the fact that it can break through defensive Pokemon with Choice Specs and Magma Storm sets. Substitute + Pain Split Gengar also works well with Swampert. In addition to its ability to switch into Breloom's Grass and Fighting combination and into Close Combat from Infernape and Lucario, it can take advantage of Clefable by setting up a Substitute on a move like Knock Off, which can put the opposing team in a very bad position.
Swampert also appreciates Rapid Spin support from Starmie, Tentacruel, and Forretress. Starmie is probably the best option because it is the most reliable spinner in DPP OU; it can check foes like Breloom, Taunt Gyarados, and Suicune, and wear down physical walls like Skarmory and Rotom-A. Swampert appreciates the support of other bulky Water- and Ground-types as well because they can act as emergency checks to Pokemon such as Flygon, Tyranitar, and Metagross, should Swampert go down to a powerful Choice Band-boosted Crunch from Tyranitar or a sneaky Grass-type attack.