Specs Kyogre and Life Orb Marshadow make a fantastic pair in Gen 8 Anything Goes. Kyogre’s checks include Kyurem-White, Eternatus, Gastrodon, Chansey, Blissey, Ferrothorn, Lapras, and Palkia. Marshadow can give a fantastic punch to all of these, OHKOing or 2HKOing every one I named. If you predict a switch to one of Kyogre’s checks when you have Kyogre up, you can send in Marsh on that turn, and there’s very few things that can switch in to Marshadow. Marshadow’s switchins include Defensive Xerneas, Toxapex (which can be broken with Bulk Up), Zygarde (which can be broken with Ice Punch), and Eternatus (switches in to close combat, but this can be predicted and you can use one of your ways to break it including Bulk Up, Max Hailstorm, or Poltergeist, and isn’t often used with Kyurem-White). Kyogre greatly threatens all of the ones mentioned.
About the Eternatus matchup, Kyogre can break the 252 HP - 164 SpD - 92+ Spe EV spread, while Marshadow can break the 252 - 212+ - 44 Spe spread.
Bulk up gives you a better matchup against stall, specifically against Toxapex which needs Bulk Up (or Dynamax on ice punch) to be broken. You can also use it on a predicted switch to 92 Spe Eternatus in to a close combat, and can be broken by a poltergeist followed by a shadow sneak. Ice Punch nails Zygarde, and Max Hailstorm KOs Eternatus with minor chip.
Once Kyogre has it’s checks down, it can absolutely wreak havoc on opposing teams, the slower the teams the better. The pair struggles against Zacian-C, so support from Necrozma-Dusk helps (plus, NDM can paralyze Eternatus to let Kyogre and Marshadow break it more easily.)
