Moves
Toxic Spikes lets Yamask support Pokemon that use Hex or stall enemies out. Yamask is able to block attempts to Rapid Spin away Toxic Spikes. Will-O-Wisp cripples physical attackers and is easily spread throughout the opposing team thanks to Yamask's good bulk. Knock Off can be used to support the team by removing opposing Pokemon's Eviolite, Berry Juice, Life Orb, or Choice Scarf. Hex synergizes well with Will-O-Wisp and Toxic Spikes but is weak when the target is not statused. Memento allows Yamask to sacrifice itself and let a teammate set up more easily by lowering the foe's offenses. Pain Split works well with Yamask's low HP stat, healing it somewhat reliably while damaging the foe.
Set Details
Mummy cripples Pokemon that rely on their ability and make contact, such as Mienfoo. Eviolite is chosen over Berry Juice to give Yamask even more bulk and to let it take Knock Off better. Maximum Defense investment is used to capitalize on Yamask's solid Defense stat, while 76 Special Defense EVs and a Sassy nature hit an Eviolite number to give Yamask maximum bulk. The remaining EVs boost Yamask's HP and Attack by one point each. If Knock Off is not used, an EV spread of 52 HP / 236 Def / 156 SpD / 36 Spe with a Calm nature is preferred.
Usage Tips
Set up Toxic Spikes once grounded Poison-types that can absorb it are removed. Spread burns throughout the opposing team to cushion the blows of physical attackers and wear down walls. Remember that you can't burn Fire-types and that you will give some, such as Houndour, a potentially lethal Flash Fire boost by using Will-O-Wisp on the switch; in these situations, predict the opponent's switch to the Flash Fire user and either double switch to an appropriate counter or use a different move. Yamask should switch into physical attackers that don't carry Knock Off and proceed to cripple them with Will-O-Wisp and Mummy, as it can easily wall them; keep it away from Pokemon such as Pawniard that can Knock Off its Eviolite, though. Don't switch Yamask into special or mixed attackers, which prey on its weaker Special Defense, can usually outspeed it, and aren't crippled too much by Will-O-Wisp. Memento should be used mid- or late-game when Yamask is no longer needed to let a partner switch in and set up on the weakened foe. Keep in mind, however, that it can only be used once and that it sacrifices Yamask, so there must be a Pokemon on your team that can take advantage of it.
Team Options
Partners such as Lickitung, Pawniard, and Tirtouga appreciate Yamask's immunity to Fighting-type attacks, which they are weak to. Lickitung provides Wish support and is a specially bulky Pokemon to fall back on if a special attacker proves troublesome for Yamask; Pawniard handles threatening Ghost-types such as Gastly with Knock Off; and Tirtouga can set up on a foe crippled by Yamask's Memento, Will-O-Wisp, and Mummy. Other Wish users, such as Spritzee, Knock Off users, such as Vullaby, and specially defensive Pokemon, such as Munchlax, can be good partners as well. Pokemon that can absorb Knock Off, such as Mienfoo and Trubbish, make for good partners. Mienfoo can also take on Dark-types for Yamask, while Trubbish provides Spikes and Toxic Spikes support, which synergizes well with Hex but can interfere with Yamask's burn spreading. Finally, entry hazard removers such as Archen and Staryu, in addition to Heal Bell or Aromatherapy users such as Chinchou, can help keep Yamask from being worn down.