Seconding TrickRoom's suggestion to use Grass Arceus over Gastrodon, with this set:
Timid; 252 HP / 108 Def / 36 SpD / 112 Spe
Your team has several issues, namely Kyogre (Gastrodon does nothing to SubCM Ice Beam versions and can't deal with Specs Ice Beam), Palkia and other dragons, and Rayquaza. Defensively, Ho-oh and Lugia can cause issues. This Arceus will patch up several of those.
Grass Knot deals with Groudon and Kyogre equally well, and Arceus can take any hit either of the two dish out. It also avoids being OHKOd by 438 Specs Draco Meteor almost all the time, allowing it to Recover stall the attack out. The defense EVs let it survive Rayquaza's +1 Jolly LO Outrage as well and strike back with Ice Beam. This is important as Rayquaza's Air Lock entirely negates Dory's sweep. Finally, Stealth Rock helps hamper Ho-Oh and Lugia, who give Doryuuzu and Landlos some issues.
With Grass Arceus covering the weather Ubers, you really have no need for Shandera, especially as it gives you a third scarfer (which is waaaaay too many, IMO). I suggest you replace it for a strong mixed attacker or wallbreaker depending on your needs. Dialga or Giratina-O work well here, the former having ample opportunities to switch in with its typing and bulk and the latter having access to a priority Shadow Sneak.
In-Game Name: Anthony
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