It's not impossible, I went very close to it by winning league, Champion's League and losing the cup final to the penalty kicks. I never bought a single token, I only win them with the offers.

You just have to manage the team the best way you can do, I usually do my best for one season, then take a rest during the next one without spending packs, tokens or money. Then, wake up early and get the best players I can in the auction

I still have my same team from 3 season ago with only a few change (2), mostly 18 years old that a trained a lot.

I don't feel it impossible, just do your best. I know that this is not the best game where you can rely on luck however