I should probably prepare for this battle in the same way. Thankfully I have Synchronize Ralts of every nature, so that's set already!I have to think about how to approach it. There is no way my capture Smeargle can take a sneeze from Primal Kyogre, let alone STAB Rain-boosted Origin Pulse. Hmm...my Vaporeon* can take hits from it all day (Origin Blade does nothing to it and Ice Beam and Body Slam are covered by Leftovers). Maybe Mega Gallade with Hypnosis and False Swipe could do the trick.
*If you think I'm not bringing the good stuff to catch it, you are insane.
There are always shinies. That's what I save my Master Balls for.