My plan is not complete, but here's what I have:
Your Team : Politoed / Tornadus / Jirachi / Toxicroak / Keldeo / Terrakion ( Focus Sash SR )
Opponents Team : Tyranitar / Latias / Jellicent / Forrtress / Starmie / Terrakion
My goal is to eliminate all counters to my sweeper, Keldeo; this would be Starmie, Terrakion, and Latias. Starmie and Latias would be my main threats; Starmie because of Thunder/Thunderbolt (of which I have no resistances; in facing Starmie I may have to sack someone), Rapid Spin and its speed, and Latias because of Dragon Pulse/Draco Meteor, Calm Mind and Psychic/Psyshock (vs.Toxicroak and Keldeo); if Terrakion happens to have EQ, he's also a threat, although that seems uncommon.
Focus Sash/SR Terrakion seems like a suicide lead, so I would set Stealth Rocks; I would expect that to lure Starmie/Forretress in sometime; if it's Starmie, I would attempt to use Jirachi, assuming it has Thunder/Thunderbolt, to take Starmie out. Toxicroak and Keldeo could face Jellicent (SuckerPunch and HP Ghost). Keldeo could take out Terrakion, Tyranitar, and Forretress(with Surf or Hydro Pump). I'm not sure how I would take out Latias, but if I am sure it does not carry Psychic, I would counter it with Toxicroak (Sucker Punch).