dw i figured it out :D

oh yeah you need to be level 40 i think level 30 will be do with your friends

Like u use them and then the end rewards of the lvls are one of those things, u can get the character if u lucky enough or just get trash

It is for a type of commission

but i cleared it at level 13 , with character at level 8 , though i had to retry 6 times

i think you have to be higher level to use secret letters. reddit is saying lvl 40.