If you have a good LC moveset for Yamask, post an answer below and upvote the best ones. Remember, this is for competitive movesets (specifically for LC), not in-game. Ability, EVs, etc. should be included. Make sure to read all the guidelines here.
Yamask Pokedex & learnset for reference.
252 HP/ 100 Def/ 120 Sp Def/ 36 Sp Atk
- Calm Mind
- Toxic/Toxic Spikes/Will-O-Wisp/Destiny Bond/Rest
- Shadow Ball
Hope this helps.