>>26716858Black tower is great and you get to fight a unique trainer at the end of it as well as receiving a pretty sweet reward if you're victorious.
Also, reminder that you get charms increasing the likelihood of shiney Pokemon encounters , less time breeding etc, when you complete national and international dex.
Even though it isn't post game you should spend time on maxing out your shopping arcade/mall too as if you are looking to complete you shiney dex, and all formes dex there are a number of shops there that will be very helpful in this and speed things up.
