Update 1.33 for AFK Arena has added a new event “Sun Wukong – King of the Monkeys”, in which you can get this hero for free.
Go to Renhorn -> Library -> Unions.
Click on any union.
If there are already maximum heroes in an alliance, replace the hero in your alliance with a weaker version, after which the “Share” button will appear.
If you don’t have the maximum heroes in your alliance, add the missing hero to your alliance, and the Share button will appear.
Now press the button twice, and Wukong is yours!
Note: sometimes the game just slows down and the Share button does not appear. Just repeat the above steps 1-2 times more until it appears. And you don’t need to share on Facebook to get the hero. Just press the button 2 more times.