Like a previous poster said, reroll your daily quests to get one that gives diamonds. You can usually get those through friendly matches as well.
There used to be hundreds of us that made rank 25 each week. Currently, there are 10 players who made that each week since it started. It's only worth 10 diamonds, so I'm sure it isn't a big deal for most of them anyway. I'm #17 on the leader board and I usually get stopped at level 22 or 23.