As usual, the Ice-type is very much a mixed blessing. Ice-type attacks are always useful on a team, but the STAB boost on them comes with an unfortunate list of weaknesses. Stealth Rock hinders Walrein's ability to tank attacks, as taking 25% damage each time it comes in will wear it down fast. Even with these drawbacks considered, Walrein is a fine tank in OU play with handy resistances to Ice- and Water-type attacks and respectable offensive stats for a predominantly defensive Pokemon.
Back in ADV, the sight of Walrein in OU matches was rare, to say the least. However, with its cool new Ice Body ability, it can be a very useful—often integral—part of a hail-based team with support from Abomasnow. Walrein's stalling abilities with hail support are impressive and with Toxic or Toxic Spikes it can be surprisingly difficult to take down. Overall, Walrein got some very useful boosts in the transition from ADV to DPP, making it suitable for use in the OU environment.
This set is based around taking advantage of Walrein's Ice Body ability and using it to stall turns.This set needs to be partnered with Abomasnow to get a constant stream of hail going. With Ice Body activated and Leftovers equipped, Walrein can alternate between Substitute and Protect, without losing any health, forcing the opponent to suffer up to 32 turns of hail and other residual damage.
Poisoning the opponent is key to this set's success. Once Walrein has afflicted an opponent with Toxic, the stalling can commence. Simply alternate between Protect and Substitute (there is no net change in your health), healing yourself and intensifying the poison damage each turn, draining your opponent of its HP. This is the direct approach. There is another way to do this, however, but it requires some additional support. Adding a Toxic Spikes user such as Forretress or Tentacruel into the mix can really add to this set's effectiveness. Bear in mind that if you are unable to get both layers of Toxic Spikes in play, you will be inflicting the weaker variety of poison, making stalling the opponent out a slightly slower process.
With Toxic Spikes support, Walrein can afford to drop Toxic from its moveset. However, if you keep it, Walrein will be able to poison Flying-type Pokemon and Pokemon with Levitate. Other than Toxic, you have a couple of options to use in its place. Roar works if your team is in need of a phazer and also has applications more specific to this set. First of all, you can shuffle opponents around to let Toxic Spikes take effect. Secondly, you can prevent Swampert from Roaring your Substitute away by Roaring it out first. Having Roar also means that Gyarados cannot simply Dragon Dance in Walrein's face and forces it to take even more residual damage upon re-entry. Earthquake is a great choice to hit Poison-types like Tentacruel that like to ruin your set-up by coming in to absorb Toxic Spikes. It also helps to wear down Steel-types, which are immune to poison. Super Fang can also be used to slice the opponent's HP in half. This ultimately makes the stalling process much faster with Toxic Spikes down. Finally, you have your choice of STAB move so Walrein's respectable Special Attack doesn't go to waste. Surf helps to wear down Tyranitar, whose ability to change the weather is a real pain. Surf also hits Steel-types for neutral damage and has the potential to 2HKO Forretress, whose immunity to Toxic and access to Rapid Spin can ruin Walrein's strategy. It also has a lot more PP than Blizzard, meaning that Walrein won't be quite so vulnerable to Taunt. Blizzard is still a worthy option, however, as it provides Walrein with good attacking coverage and smacks around most Flying-, Grass-, and Poison-types many of whom are immune to Toxic Spikes or the poison status altogether.Team Options & Additional Comments >>>
Walrein has a few options worth noting from its respectably diverse movepool. Stockpile is available if you wish to boost both of Walrein's defenses, although Walrein has better things to be doing. Hidden Power Electric works okay alongside Blizzard, hitting Water-types for reasonable damage, especially Gyarados. Walrein can run a RestTalk set with Thick Fat, but its poor defensive typing and weakness to Stealth Rock make the set's viability questionable. Walrein can also run a Curse set to further boost its decent Attack and Defense, but Snorlax and Swampert do this job better. The only distinct advantage Walrein has over the former two is that nothing is immune to Avalanche. Lastly, Encore can be used to force switches and set up on the opponent's Pokemon. Roar is probably the better option, though, since it has more PP and can bring out something Walrein fares well against. Thick Fat can replace Ice Body pretty much anywhere, but then you're better off using other bulky Water-type Pokemon such as Vaporeon or Suicune. Brine is also a nice attacking move in tandem with Super Fang.
Checks and Counters
Since Walrein relies on poison to deal damage, most Steel-types will cause problems. Jirachi and Metagross have the defenses to take a few hits even from Earthquake and strike back with powerful attacks of their own; however, unless they carry Leftovers, both of these lose in the long run against Walrein. Empoleon can also Roar away Walrein's Substitute, but rarely has enough Speed EVs to ensure it will phaze Walrein before being phazed itself. If they come in before Walrein sets up a Substitute, Lucario and Machamp can scare away Walrein with their high-powered STAB moves.
Tyranitar screws up Walrein's Leftovers recovery and replaces hail with sandstorm, making Ice Body useless. Gyarados can sponge hits from Ice- and Water-type attacks and set up with Dragon Dance before KOing Walrein with Stone Edge. Like Tyranitar, Gyarados also has access to Taunt. Suicune is faster than Walrein, can easily Roar it out, and takes minimal damage from any of the Walrein's attacks. Clefable is immune to the effects of hail and poison and can Encore Walrein's attacks, making it particularly problematic despite being rare in OU. Breloom is another issue if you don't have Blizzard, since it, like Clefable, is still able to recover HP during hail and can't be worn down by poison, thanks to its Poison Heal ability.