In the Fyber offers, look for the one for Legendary: Game of Heroes. Reach Rank 20. Proceed to complete Level 1-2 over and over and over. It's 5 energy with an exp rating of 200, it's a long grind, but it's super simple and easy to perform.

Update: So, completed at 1:12 and checked my mailbox. As of 1:12 I was rewarded the 3856 diamonds and 771 VIP points.

If you're looking for something to do while waiting for anything to refill in DH, I'd recommend this, it's tedious, but it's a large amount of diamonds for free when you're killing time. I personally didn't like the game, but if you're into the Puzzles and Dragons type games, you might enjoy it.