Moves
Substitute allows Wailord to block status and scout switch-ins while letting it also PP stall moves. Aqua Ring gives Wailord recovery that can be utilized with Protect and Substitute. Protect gives Wailord a free turn of recovery, which can also be used to scout a foe's moves. Scald deals chip damage to a foe while also having the chance to burn it.
Set Details
252 Defense and Special Defense EVs maximize Wailord's overall bulk, allowing it to set up on as much of the metagame as possible, including resisted Choice-locked moves such as a Waterfall from Floatzel. Pressure allows Wailord to PP stall the majority of the metagame when set up, including threats such as Calm Mind Clefairy. Leftovers gives Wailord a bit of residual recovery, which, when paired with Aqua Ring, allows Wailord to lose no HP when using Substitute and Protect to PP stall a foe. A Bold nature is viable, although a Calm nature is generally preferred, as it allows Wailord to take special hits better, as most Water-types are special attackers.
Usage Tips
Wailord should be used as a bulky Water-type to absorb resisted or weaker neutral hits and as a nearly unbreakable PP stalling wall after it has set up. If you notice that Toxic Spikes can be easily set up without them being absorbed by a Poison-type foe, that can be an indicator that you can use Wailord as a wincon late-game. Wailord should avoid being inflicted with burn or poison, as it will be unable to survive for long periods of time if it loses HP every turn. Wailord should be preserved to PP stall a threat to stall teams, such as Duosion. Be wary of Boomburst Chatot and Infiltrator Jumpluff, both of which can hit through Substitute. Wailord needs entry hazards set up on the opponent's side of the field to be effective, as foes can switch around without much trouble otherwise, limiting its PP stalling abilities. Don't be afraid to use Wailord's natural bulk to switch in on threats it can wall, such as Camerupt.
Team Options
Wailord requires support that generally only stall and semi-stall teams can provide. Toxic Spikes is almost mandatory to allow Wailord to stall out Pokemon with Substitute and Protect more easily, saving its own PP. Two layers of Toxic Spikes, which badly poison foes, are often better than one if your opponent has a Pokemon with reliable recovery, such as Milk Drink Gogoat. Roselia is the best option for a Toxic Spikes setter, as it has the bulk to set them up multiple times in a game and resists some of Wailord's weaknesses. Entry hazard removal is extremely valuable to keep Wailord healthy, as Toxic Spikes completely cripples it, and Stealth Rock and Spikes make it harder for Wailord to set up; thus, Rapid Spin users such as Avalugg and Armaldo and Defog users such as Vullaby and Vibrava make for great partners to Wailord. Misdreavus and other Ghost-types can be used as effective spinblockers. Cleric support is also extremely helpful for Wailord, giving it another chance to stall again if it is inflicted with status, such as an unfortunate Scald burn. Lickilicky and Audino have access to Heal Bell and are able to easily sponge special Grass- and Electric-type moves for Wailord. Toxic users help Wailord stall out more foes that are immune to Toxic Spikes. For instance, Gabite can use Toxic on many Defog users that switch into it, which allows Wailord to PP stall them more easily late-game. Wailord pairs well with Pokemon such as Roselia, Camerupt, and Klang, as they can take on Grass-type Pokemon with ease.