How about a clear answer for a change:
When you press Top-50, you randomly fight against the deck of a player who has one of the 50 highest point totals. The AI controls that deck against you, not the other player. The player's deck can change constantly; every time he logs into Elements the deck is saved ... so for example some players put up farms (decks that have a lot of rare cards but no way to defend themselves) just before they go offline, and then take them down and use their normal decks when they play again.
Generally you will need a score of 150,000 points to be in the Top-50 at all; or around 200,000 points to be there for a long period of time. Top-50 eligibility is determined by your total score, not by what you've scored in a given time period. However, it is RESET every Sunday and starts recollecting scores. This happens when players RESUBMIT their scores by clicking the "highscores" button from within the game. Since many top players like keeping their decks in the Top-50, they wait for the Sunday deadline and make a habit of pressing the button immediately. So if you have a relatively low score (under 100K for example) you really are not going to have much chance to get into the Top-50 for more than a few minutes.
Top-50 is especially good as a player for collecting rare cards because there are often farms up. The best way to do this is to head into the Elements chat and look around for Top-50 players. Often several will be hanging out there. You can ask them and often they will be nice enough to build a farm for you right then and there, putting in the rares you still need to collect. Having done that, you can spend some time "farming" the Top-50 and hoping to fight their decks.
You will notice that occasionally one player has several copies of his deck within the Top-50. This is done by logging into the highscores from several different machines and can be a way of greatly increasing your chances of fighting a farm. There is no rule against this practice, however it is generally frowned upon by the community.
Note that the Top-50 feature's time is limited. Zanzarino, the game's developer, intends to replace the feature entirely. Speculation has it that that will happen next Spring or Summer.
