This is very easy to get by yourself if you have 2 iOS devices (e.g. an iPhone and iPad). You also need 1 diamond in your inventory.
Ensure that both devices are connected to your wifi, and on the host device, make sure Multiplayer Game and Broadcast to LAN are enabled in the options screen before loading the game.
Use your second device to join that game and wander over to your host character.
Note that "throwing diamonds at another player" simply means dropping a diamond on the floor and allowing the other player to pick it up. Repeat again to get your diamond back.