Thank you, Paul !! I was also thinking that it should be rather "billiard tricks" ...Although it depends on the context. Maybe they are both possible in singular and in plural form. Just one phrase is more common than the other one. How about a phrase ''Pool trick shots'' Or, ''billiard trick shots.'' Does it sound natural in American English?

Paul, in actual usage billiards in this context is much more common than billiard. Do a google search in the singular and the plural. There are far more hits for the plural.

"billiard tricks" would be the answer. Just remember that the noun is "tricks" and the word "billard" is functioning as an adjective. In English, we don't concord adjectives for number or person. Thus billiard remains in the singular. In other words, grammar imposes that it be singular, even if the name of the game, which is a noun, is in the plural i.e. Billiards. In any case here is an example. " My friend know some really cool billiard tricks. Another classic example of a similar type of problem would be e.g. Mary is thirty years old.....or Mary is a thirty-year-old girl. Notice the word "year" in the first example is "years", with a final s, but in the second example is in the singular "year" because in this instance it is functioning as a adjective not a noun