Would someone be willing to please post sets for all tiers (based on pet ability for each opponent. For example: set advice wouldn't be needed for a T5 for an opponent that's beatable by a T1) for the optional plot opponents please? I tried to beat an opponent that had way fewer stats than one of my battlers and I lost miserably.
If someone's willing, an affordable and best sets would be wonderful please

Which opponent are you specifically trying to beat? Organ Collector is beatable by tier 4 pets and Umbrus by tier 6 pets. But to do so at those tiers is not cheap. If your pet is a higher tier you can afford to spend less but it's hard to advise without knowing the opponent or tier :)
I have a T5 pet and she beat Organ Collector only twice, now she cannot beat him. I am ready to upgrade everything (am almost T6). What should I get please?
Specifically against Organ Collector the Massaquerade and Hearts Revenge Gunblade are a good combination that I know other pets have had success with. They are Tier 5 or under so you can use them now and get yourself some organs. General T6 advice most people go for the Reborn Icewand of Doom as their main weapon which is 20m if you can buy both parts from the Millionaire Center. Replica Minotaur Knocker is useful until about Tier 8 if you don't have one already. If you have a reliable freezer, one or two healers (Phoenix Quill, Reviver of the Lion) and a blocker (Rainbow Vortex, Shinwas Temple Shard) then most opponents shouldn't be any trouble :)
what are the 2 parts you canbuy in the Millionaire Center for the Reborn Icewand of Doom? And thank you for all your help and advice
They are and The Freezewand you can get in ~2 months from now. The Reborn Wand won't stock for another ~8 months though :( So if you can find a cheap one selling then keep it in mind. Otherwise you can use the Replica Minotaur Knocker along with something cheaper like the Massaquerade or a Blunderbuss and hold off on the next big purchase until Tier 7 or Tier 8.