On the Calm Mind set, make sure you stress that Water is not a great attacking type in Ubers. Water Arceus will usually only be able to set up late game, since most teams will carry multiple Pokemon that can check it effectively. Also mention a Physically Defensive spread in Set Options (told you to do this already, see my first post in this thread for reasoning).
Move Grass Arceus up on the Checks list behind Zekrom, and Mega Gengar down behind Shaymin-S. Standard Gengar will usually need to sacrifice itself to kill a 100% HP Water Arceus with Destiny Bond since it cannot 2HKO whereas Water Arceus 2HKOs in return. Mention the Sub-Disable-Perish Song set fares better against Water Arceus than standard. Taunt Mega Mewtwo Y and regular Mewtwo can check Water Arceus as well due to their good Special bulk and ability to stop recovery. Move Mewtwo and its formes between Shaymin and Gengar.
Move Grass Arceus up on the Checks list behind Zekrom, and Mega Gengar down behind Shaymin-S. Standard Gengar will usually need to sacrifice itself to kill a 100% HP Water Arceus with Destiny Bond since it cannot 2HKO whereas Water Arceus 2HKOs in return. Mention the Sub-Disable-Perish Song set fares better against Water Arceus than standard. Taunt Mega Mewtwo Y and regular Mewtwo can check Water Arceus as well due to their good Special bulk and ability to stop recovery. Move Mewtwo and its formes between Shaymin and Gengar.