Ninja Gaiden Sigma Plus

Getting platinum coins requires you to clear a mission on Master Ninja difficulty.

Normal = Bronze coin
Hard = Silver coin
Very Hard = Gold coin
Master Ninja = Platinum coin

There are a total of 76 missions. This will probably your last trophy to unlock along with Thanks for playing! - from Team NINJA.

You can check out how I completed most of the trials on Master Ninja difficulty here:  https://www.playstationtrophies.org/forum/ninja-gaiden-sigma-plus/150128-ngs-mission-mode-all-missions-completed-master-ninja-difficulty.html