Posted on 03 May 18 at 15:11, Edited on 03 May 18 at 15:15 by Neltharion2010
You only need 3 keys (from the 5 available trials at this point), which you get by beating any of the Impossible-Difficulty Trials.
After collecting 3 keys and then beating the 6th trial for the first time, the trophy will pop. It is not necessary to beat every trial on impossible difficulty! Just pick the 3 that are easiest for you.
Trials #2, #3, #5 are recommended, they are the easiest by far.