Round three. FIGHT.
Alright, looking at your team so far Politoed/Tentacruel/Thundurus is a solid start. Swampert isn't the best in this metagame, as its offensive stats are lackluster and its defensive stats also don't allow it to take many hits. I'd recommend using Sub CM Jirachi instead of Swampert. It takes your two uncountered weakness (Rock and Psychic) with relative ease, and it hits hard after a couple of boosts. Substitute and Calm mind are the two obvious moves of this set, but the offensive moves are a bit harder to choose. Psyshock, Thunder, Flash Cannon, and Water Pulse are all solid choices. Given your team's offensive presense so far, Psyshock and Flash Cannon seem like your best options. However, feel free to test out all the coverage moves.
Looking at your team now, you have no physical presence. You also have no resistance to Ghost and Dark types, and a rather large weakness to Electric and Ground attacks. Finally, you have no entry hazards, which is a bit of an issue. I feel like Landorus-T is a solid fit for your team. A set of Stealth Rock/Earthquake/Stone Edge/U-Turn, but you can use other moves such as Toxic or Gravity over U-Turn depending on how offensive or defensive you want to be.
Finally, you'd appreciate a late game cleaner and a dedicated offensive sweeper. I feel Lucario is a solid choice for your team. A moveset of Swords Dance/Close Combat/Extremespeed/Ice Punch, Crunch, or Extremespeed would serve you well. He would also provide a valuable resistance to Ghost and Dark attacks.
I hope this helps you out!! This was off the top of my head, so there may still be issues that I missed. Good luck!
