As explained in the game, the ways to get diamonds in the game are those that can be achieved using the application itself, either through gifts from friends, in the case of playing in duos or squads, or by fulfilling certain missions that are rewarded with some small amounts of diamonds. The latter is rare, but if it happens you should take advantage of it.
However, you can also use other methods outside the application itself. These need legal, ban-free apps or websites. One of the ways is to look for ways to win free cards in Googe Play and you can do that in different ways such as: downloading a certain game, doing surveys, completing missions, etc.