1. Cube
cube rewards is including diamond, sometimes even wood cube can give you diamond (3 or 5 or 7) sometimes can up to 10 or 15. You can get more by opening gold or diamond cube.
2. Event
It's actually quite often Let's Get Rich is having event. For example : MAX A+ character and get 200 diamonds or get S pendant and get 200 diamonds.
3. Diamond Class
This is the fastest way to get diamond i think, rather than waiting for event or opening hundreds of cubes.
But this was is also risky bcoz you may lose 20 diamonds if you lose.
But thats if you lose, if you win, you can get diamonds plus golds, 30 diamond - 20 diamond (for ticket) = 10 diamond per win plus 1100 gold bonus and random cube.
Furthermore, if you win from triple, line or tourism, you can get gold up to 15k - 20k
I already got hundreds of diamonds and gold from this way.
One more thing, you can save some clover because diamond class doesn't need clover.
4. Ranking Reward
And this is the easiest way to get diamond, buy you can only get it once a week
See the picture, with score around 190.000 you can enter top 6% which gives you 35 diamonds + 1 premium pack as rewards. To get into rank 1000 or above you need at least 1,8 ~ 2M score or above. For me, 1% is enough, with score around 500k you can already get into top 1% if i'm not mistaken (sorry i forgot the exact score)
Hope this tips are helpful