The Top 50 is reset every week. Players who post their highscores using the button in the game are recorded in the Top 50 if their scores are high enough, and when they press the button, their current deck is also recorded... it then updates every hour or so to keep their deck current. Some generous Top 50-qualifying players decide to post decks which have only a couple types of cards, are impossible to beat you with, and contain only rares and/or pillars.
If you run into one of these decks when playing the Top 50, you are very likely to win at least one rare card - sometimes you can win multiples, I've won three rares in one game before. They are posted at players' discretion and are a favor to the community, not a guarantee nor are they obligated to do so. Players can edit the initial post of this thread to keep the spreadsheet there current, so other players can know in advance what rare cards might be available from playing the Top 50. You can also post in this general forum if there's a specific rare card you're seeking - Top 50 players may or may not read your request, and may or may not follow it, but it's a shot.
The Top 50 will soon be changed to the Top 500, and it's uncertain if it will be practical or even possible for Top 500-qualifying players to set up such farm-able decks, so if you wish to trawl the Top 50 for rare cards, best to do it soon! There is a Top 50 section on the main page of this forum that also contains good decks for farming the Top 50... while true rare farms are nearly impossible to lose against, a good Top 50-oriented deck will ensure you don't waste vast amounts of coin against the non-farm decks the Top 50 contains.