Grand Master Yoda is a very powerful Jedi who works well as a team leader, if you plan to use a full squad of Jedi Knights. Acquiring him is only possible during the rare Grand Master's Training event, which occurs every two months (alternating with Emperor Palpatine) and lasts for a week. The event consists of 7 levels and on each of them you have to use a team composed of Jedi Knights. 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 Grand Master Yoda is only possible after collecting 145 shards, 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 progression of character acquisition is saved, so even if you do not finish all the fights during the event, you can continue unlocking Grand Master Yoda in two months.