Gen 6
Walrein @ Leftovers
Ability: Oblivious / Thick Fat
EVs: 248 HP / 82 Def / 180 SpD
Calm / Sassy Nature
- Toxic
- Aqua Ring
- Protect
- Surf / Super Fang / Dive / Whirlpool / Stockpile
This moveset works wonders. Toxic is your straight forward way of dealing damage as Walrein is an extremely stally Pokemon. Aqua Ring is for 18% recovery each turn when coupled with Leftovers. Protect is for stalling Toxic + Aqua Ring turns. For the last slot, you really have a lot of options. Surf is for STAB, Super Fang is for halving the opponent HP, which makes it rather easy to KO the target, Dive is for stalling out a turn + damage (if running Dive, run a Sassy Nature as going last is always appreciated when using Dive), Whirlpool is for dealing even more damage per turn to the opponent (if running Whirlpool, use Thick Fat as it is not legal with Oblivious) and (finally) Stockpile is for setting up defenses and goes well with Oblivious as you cannot be taunted. Oblivious, as stated before, is highly recommended for Walrein as you cannot be taunted and also because it is it's only niche as a staller in PU, and Thick Fat is so that Whirlpool can be legal, but also has its own niche in reducing damage taken from Fire and Ice type attacks. If running a hail team, you should opt for Ice Body, as this gives you 3 recoveries per turn (until Hail ends), which is outrageous.
edit: Trachy has informed that odd HP EVs are better for Stealth Rock and Spikes, so I have changed Walrein's HP EVs to 248, then dumped the remaining ones into Defense. Thank you!