This is the one trophy that might take you the longest. The 21 Class Leaders are those that you can find on the Causality Link with a vibrant spike barrier around their portrait. To get to these leaders, you must forge friendships with their peers and create a link to them. Any student you come across that does not have a 3 affinity, talk to them to open the possibilities up. Once you've got the Leaders available, hunt them down and then listen to their traumas, have them join the party, and then solve their issues. There are two leaders that might be challenging:
2-5's leader requires all connected to have star-affinity. If you look at the Causality Link for them, they only have two. The one to the southwest of the leader is easily resolved, but the one north says that you need to resolve the trauma of all connected to it; this is false, as only one was solved and I managed to get the student to star affinity from that. So focus on the northeast student's plight rather than Yui's.
2-7: Hallucinosis is a pain in the butt to try and resolve traditionally. You're required to take down two targets, rather than the intended one. Hallucination is in Grand Guignol, a Lv.70+ Digihead surrounded by two others. Normal means may not allow you to properly take them out; thankfully, you can change the difficulty over to Easy in Options and even if there's a twenty-level difference for you as there was for me, it'll allow you to overcome it if you are wise with your placements of abilities. Do not fight on Auto, so you can actively guard when necessary. My party was MC, Izuru, Kotono, and the CL for the sake of resolving the trauma. However, after finishing off the Hallucination, you'll have to go fight the Lord of Mirage afterwards. That fight is a little bit easier.
A full list of all students, where to find them, and what their traumas are can be found here:
https://docs.google.com/spreadsheets/d/1885VeLVlmiNgc7JjhH8Q...