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.
As previously mentioned, Abomasnow is absolutely required in order to take full advantage of Walrein's ability. Since Walrein is normally used with hail, Abomasnow could support the entire team as a whole. Toxic Spikes will allow Walrein to more effectively stall out the opponent, so partnering Walrein with a user of them is a good strategy. Tentacruel and Forretress can reliably place down Toxic Spikes while also providing Rapid Spin support. Rapid Spin removes Stealth Rock from the field, allowing Walrein to come in without taking a quarter of its HP in damage.
232 HP EVs ensure that Walrein's HP stat leaves the same remainder when divided by 16 and 4, which guarantees that all of the damage it takes from using Substitute is restored in two turns. Bearing this in mind, the EVs give Walrein the best possible overall defenses. Speed EVs can be used on Walrein in order to foil some common phazers such as Skarmory and Vaporeon. However, this might reduce Walrein's overall physical tanking abilities, albeit not by too much. More importantly, though, not investing in Speed ensures that Walrein is slower than standard Choice Band Scizor, which would otherwise break Walrein's Substitute while switching out to a faster Pokemon, thus breaking the stall.
Tyranitar gives Walrein a world of trouble by removing hail and packing high-powered Rock-type attacks. Swampert can keep Tyranitar at bay, and can also set up Stealth Rock for added residual damage. Walrein's typing gives it an undesirable weakness to a number of commonly used attacks. Fighting-types who are immune to poison, such as RestTalk Machamp, can beat Walrein with their STAB attacks. Gyarados can also be a problem with Taunt, an immunity to Toxic Spikes, and the ability to then set up on Walrein. Having a Rotom-A on your team will compensate for these issues extremely well. Rotom-A can check and beat Gyarados, take the Fighting-type attacks aimed at Walrein, and also block Rapid Spin attempts from Pokemon like Forretress.