It is accepted. Please do use the report function (My answer should be accepted) next time, do not just write into the discussion.
What is wrong with "We would frequently/often win back then"? And why is the continuous tense used in the proposed solution? Shouldn't simple tense be used with words like often, frequently, seldom, always etc.?
"We would win frequently back than." is already accepted. I do not know whether your word order is the same.
I do not think that the continuous tense with frequently is wrong. If you do, please tell us why, show some reference or some other reason.
No, it is strange. Často/often means it was repeated so you need the imperfective verb. You can say "Tehdy jsme vyhráli." for "We won back then.".