Gastrodon

Gastrodon is a premier counter to one of the strongest Ubers Pokemon in Kyogre because it's immune to Kyogre's STAB attacks and Thunder. Unlike Eternatus, Gastrodon is less likely to lose to Kyogre through status and can always beat Kyogre's Choice Specs set. It can also switch into Dracovish spamming Fishious Rend, safely switch into defensive Necrozma-DM not running Knock Off, and counter special Palkia and Dialga that are not running Choice Specs. However, Gastrodon is a more passive switch-in to Kyogre compared to other options like Eternatus and Ferrothorn and lets it freely use Rest; unlike Gastrodon, other special walls are able to switch in and shut down Kyogre by using it as an opportunity to heal, inflict status, or set entry hazards. Gastrodon is also susceptible to all entry hazards, especially Toxic Spikes, if not running Heavy-Duty Boots. When not facing Kyogre or Dracovish, Gastrodon is extremely underwhelming and offers little defensive value.