just level other random classes without giving a care to win or loss. you get a lot of free cards for every class from doing it. you likely aren’t doing your baby quests either. i have a f2p account and got 30 levels on previous r rewards track without any sort of boosts otherwise in a day or so of playing, because the game vomits free stuff at you at first. haven’t logged into it in a while though.
https://hsreplay.net/collection/1/566411825/
notice i have 15 legendaries plus other stuff. i don’t have all core cards (cuz i haven’t played at all). this account is at apprentice 40 or whatever it is that just popped up. haven’t even made it to bronze yet, as i don’t play. you can also see i have almost no barrens cards at all cuz i haven’t really played it.
edit: this account doesn’t have a level 5 cheeve for any class. but i spent my gold on packs to game pity timer things and gotten the legendaries you’ve seen. i don’t have any tied to levels though i don’t think.