Although R2-D2 is not an exceptionally powerful character, any Star Wars fan will surely want him in his collection. Acquiring him is only possible during a very rare Daring Droid event, which has no clearly defined time frame. The event consists of 7 levels and on each of them you have to use a team composed purely out of Empire units. The first fight requires at least 1 star in each character, the second fight requires 2 stars, etc.
The reward for completing each fight is a specific number of character shards, that are then used to unlock the unit. Important: the activation of R2-D2 is only possible after you have collected 145 fragments, over the course of 5 fights. Each skirmish can only be completed once, so it is only possible to promote the character to 7 stars after completing all stages. The character acquisition progress is saved, so even if you do not finish all the fights during the event, you can continue unlocking R2-D2 the next time the Daring Droid event is available.