Super Magical Chests are incredibly rare, but is it possible to increase these small odds?
Super Magical Chests run on a 500 chest cycle. At some point within your first 500 chests you will receive a Super Magical. And again in the next 500 chests (once you've reached the end of the first 500 cycle). Crown chests and free chests do not count towards the 500.
To increase your chances of getting one, the only thing you can do is gem your regular chests to get through the cycle faster.
If you've been playing for ages up to now and are sure you've had more than 500 gameplay chests, the reason you haven't received one will be because when SuperCell reduced the cycle length from 1000 to 500 everyone's position in the cycle got reset to 0.
I received a SMC recently so I know it's possible and I've only been playing 3 months at most.
I learned this information from one of the many Youtubers who make Clash Royale vids, either Chief Pat, Galadan or Clash with Ash. All of the main Youtubers are in direct contact with the SuperCell team and thus we can trust their word.