Walrein @ leftovers
trait: Ice body
Evs: 232 HP / 252 Def / 24 SpDef
Bold nature (- atk, + def)
Surf / Blizzard
Toxic / Super Fang
What it does: Exactly what it looks like. Stalls teams in hail. Your primary objective is to first set up a substitute, which, with the given EVs, cannot be broken by seismic toss and night shade. Next use either toxic or super fang (toxic is the better stalling move, so I would highly suggest toxic) to wear the opponent down, using protect to piss off SubPunchers and, well, stall toxic and hail damage. Finally, finish the enemy off with blizzard or surf. Blizzard is more powerful and has perfect acc. under hail, but has low pp, so surf is more, ah, durable. Now, of course you need hail support, as this set can only function under hail (snover is really your only option, but it's really not so much dead weight as you would think). I should mention that in two turns, recovery from both ice body and leftovers allows full recovery from substitute damage.
Good teammates: Walrein requires defensive synergy such as roselia, which can switch into Super effective fighting- and electric- attacks that would just make Walrein look like a child's toy. Roselia can also absorb those toxic spikes, which would otherwise poison Walrein, greatly hindering it's durability. As I mentioned before, you need hail support for the aforementioned reasons. Seriously, don't even think about using this without hail.
What counters it: As also mentioned previously, fast, powerful fighting and electric types, including electivire, galvantula, all forms of rotom (those that are present in RU), Medicham, HOLY SHIT POLIWRATH, and even grass types such as Sceptile will quickly drain Walrein's HP with their supereffective STAB. Status also ruins Walrein, specifically burn and poison, as they induce residual damage.
Additional Info: HOLY SHIT POLIWRATH
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