First off, if you're not particularly experienced in deck-building, I would recommend checking out some
recent winning decks on the official site, and observe the cards used in those lists. Is there something used in quite a few lists? Think about why those cards might have been used. In addition to looking at these decks, PB has some
great articles definitely worth reading. This way, you can get into the heads of experienced competitive players and how they deck-build!
Unfortunately, the Primal Kyogre archetype hasn't been that great recently, so up-to-date lists may be rare. However, no need to worry! Honestly, the best thing you can do is just dive straight in (excuse the pun!) and create a decklist for Primal Kyogre basically from scratch. And it doesn't have to be perfect! Once you've created a 60-card decklist, you can post it in the
Deck Garage for further, more specific advice. (And if you do, let me know, because I'll be glad to help more specifically. =) )