Salamence and Rayquaza both share the same weaknesses -- specifically Rock, Ice, and Dragon moves -- so placing Pokemon on your team that can take such hits is pivotal.
252 Health Forretress is a good idea. It can spin away the rocks that your dragons hate so much, can take Choice Scarf Stone Edges from Terrakion and Landorus (though not repeatedly), and put up hazards of its own to make pulling off sweeps that much easier. Stealth Rock also makes Thunder Wave Parashuffler Lugia, who gives you MAJOR problems, easier to deal with.
To protect yourself from Ice Beam, Arceus-Water or Arceus-Steel would be good options. Use the latter only if you don't choose Groudon as a weather setter (I think your core is better off weatherless IMO). A status move, preferably Thunder Wave or Toxic, is a must to cripple Lugia and fast special attackers like Palkia that could give your team issues. From there, whether you choose to make your Arceus defensive or offensive is up to you...
Finally, I feel you need a special attacker. You do not want to use a Dragon that is weak to Dragon or Ice moves, as stacking types is always a poor idea. I would recommend Choice Scarf Dialga. Dialga can perform fairly well vs. weakened Extremekiller and Arceus-Dragon (who your team has some issues with). Draco Meteor, Fire Blast, Thunder (for Kyogre), and Dragon Pulse would be a good set. Max speed isn't necessary, but make sure you can outrun threats like Mewtwo, Darkrai, and Shaymin-S. Dialga also stops Scarf Terrakion from revenging your dragons with Stone Edge
My suggestions still leave you weak to certain threats, such as Extremekiller and Bisharp, so make sure you have an effective way of dealing with those! Max defense Hippowdon or Sableye can check it to a decent extent if you want to be innovative. Regardless, best of luck! I really recommend Forry for your team, but my other suggestions are far from foolproof.