This Trophy requires you to get a score of 600,000 pts in Shield Shredder by collecting as many of the purple Diamonds as possible. This can be earned after completing the world
Arendelle. To start this fast travel to
Elsa's Ice Palace and speak to
Goofy.Easiest way to do this is to try and avoid the rails as there are bigger versions of the purple diamonds on the ground. If you hit these they will cause a bunch of the little diamonds to spawn, giving you a fair bit of points. Also try avoid hitting a wall as this will knock your multiplier out.
Here is my video guide to hopefully help you with which direction to go.