I think the "mastery" is memorizing what the scoring cards are and making sure the board is not in a position for your opponent to easily score those, if they have them. Like, there is a scoring card for 3 blue tiles to be in a column together. So if you have not seen this card yet, you need to make sure you don't leave blue tiles all on top of each other and accidentally give your opponent 3 free points. If you start to recognize which tiles are scored together, whenever you have a turn where you didn't use all your cards, you can use the extra card or two to shift the board into an un-scorable position for your opponent. Like (I'm just guessing for this example), it seems like there are at least a few cards that score whites and greens in different positions relative to each other. So if you know what those positions are, and you notice there are white and green tiles near each other on the board, you could use your extra cards to move them away from each other so that, at the very least, your opponent has to waste cards to move them back to where they were. Stuff like that.