To defeat Primal Groudon, bring high-level Ground-type Pokémon. To defeat Primal Kyogre, bring high-level Grass- and Electric-type Pokémon.
Pick the Pokémon that is already the highest level to try to raise to 100. It doesn’t matter if it’s not your most powerful Pokémon, since you’ll only be using it to gain access to the special area where Reshiram/Zekrom can be found. These Legendary Pokémon are only Level 50. Training to Level 100 may take a while, but you can speed things up with a few tricks. See Get Your Pokémon to Level 100 for details.
Reshiram - Bring Pokémon with Ground-, Rock-, and Dragon-type attacks. You’ll also want a Pokémon that can inflict Paralyze, which will increase your chances of capturing it. Zekrom - Bring Pokémon with Ground-, Ice-, Dragon-, or Fairy-type attacks. Zekrom cannot be paralyzed, so bring a Pokémon that can put it to sleep instead.
If at any time during the battle the Pokémon breaks free or wakes up, use the status effect again to keep it trapped.
See GTS Barter for What You Want for tips on getting the best trade deals in online trading. There are no cheats available for Omega Ruby or Alpha Sapphire that will allow you to get the other Legendary Pokémon.