This run category consists in getting the first magical chest of your chest cycle starting with a new account. Do not worry, it is not luck based! Your magical chest will ALWAYS appear on your 7th chest, meaning that after finishing the 5th trainer, you must win 7 times.
See : This is the chest cycle of a fresh account, after finishing the 5th trainer; the magical chest is here 8 chests away, however here I believe the API was not up to date yet, as it took me 7 wins when I did my run; the first chest must be the silver chest that is given when finishing the 5th trainer.
Important! Make sure you have an empty chest slot before starting a battle, else you will not receive a chest from wining nor progress further in the chest cycle towards the magical chest! When you slots are full, just open any chest with gems to skip the 3 hour wait. You should have enough gems to make it to the magical chest, so do not worry about running out of gems.
Good luck!